Add Redis Sentinel HA docs (port from CE)

See merge request !884

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Add debug logging and sync troubleshooting information to documentation

See merge request !843

Geo: Fix cache clearing after repository update on secondary node

When replicating operations originated from `git`, we don't have the same entry point (`git_push_service.rb`) on the secondary node, and because of that we had to manually duplicate some code after the update happens on the secondary (Pull vs Push).

This MR will introduces the following changes for Geo (#76):

* cache backfilling by `ProjectCacheWorker`
   - The call will switch from normal to **Geo** specific cache building code (when in a secondary node). 
* triggers after_* repository hooks when necessary to clear cache after specific events (when in a secondary node).

Fixes #1129 

cc @marin @dewetblomerus @patricio 

See merge request !869

Move issuable title form to a new partial (EE)

EE part of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7453

See merge request !878

Update help links in Geo Nodes

The links for Geo (#76) documentation are outdated and lead to 404s. These updated links lead to 
the respective help page.

See merge request !875

Remove gitattribute entry for CHANGELOGs

This should no longer be necessary now that we've got automatic changelogs!

See merge request !874

Signed-off-by: Rémy Coutable <remy@rymai.me>

Clarify LDAP troubleshooting ldap_search example and align CE/EE docs [ci skip]

A customer noted an error/lack of clarity in the LDAP documentation
with the `ldap_search` example. Previously, if taken literally, the
customer may have expected the `$` variables to be automatically
replaced or if they paste the exact `user_filter` contents the
parentheses would have been incorrect. Let's just simply the filter
and use exactly what's in the configuration.

Also, this file was way out of wack between CE and EE. I synced them, except
for the banner at the top that links to EE LDAP docs on the EE side.

See merge request !877

A customer noted an error/lack of clarity in the LDAP documentation
with the `ldap_search` example. Previously, if taken literally, the
customer may have expected the `$` variables to be automatically
replaced or if they paste the exact `user_filter` contents the
parentheses would have been incorrect. Let's just simply the filter
and use exactly what's in the configuration.

Also, this file was way out of wack between CE and EE. I synced them, except
for the banner at the top that links to EE LDAP docs on the EE side.

Geo: fix a problem that prevented git cloning from secondary node

This MR prevents activity_service writing in secondary Geo (#76) node's database. This will fix the failure users were experiencing when trying to `git clone` from the secondary node.

cc @marin @dewetblomerus @patricio 

Closes #1243 

See merge request !873

The links are outdated and lead to 404s. These updated links lead to 
the respective help page.

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Refactor instructions for Geo docs

When I followed these instructions, it was hard to keep my place, or get a feel for how close I am to the beginning or the end, so I am trying to create a numbered list for the **Secondary Node** instructions, but doing formatting while keeping it as a list is proving to be really hard.

/cc @axil

See merge request !864

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Add a note about LDAP nested group resolution.

For LDAP sync, nested groups must fall within the configured
`group_base`. A customer recently ran in to this issue. It's
not immediately obvious but at least we can make a note in the
documentation.

See merge request !872

For LDAP sync, nested groups must fall within the configured
`group_base`. A customer recently ran in to this issue. It's
not immediately obvious but at least we can make a note in the
documentation.

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