Use task_list gem for task lists

Task Lists can now be used in comments, and they'll render in previews. 👏

Closes internal https://dev.gitlab.org/gitlab/gitlabhq/issues/2271

See merge request !599

Revert "Allow to configure smtp and sendmail in gitlab.yml"

Add SIDEKIQ_MEMORY_KILLER_SHUTDOWN_SIGNAL env var

It looks like SIGTERM may not be enough to shut down a Sidekiq process
when its RSS has gotten too big. This change will allow us to experiment
with sending SIGKILL instead of SIGTERM to Sidekiq processes on
gitlab.com.

See merge request !1812

Remove tag duplicates

Prevention of getting this error:

```
ActiveRecord::RecordNotUnique: PG::Error: ERROR:  could not create unique index "index_tags_on_name"
DETAIL:  Key (name)=(XSS') is duplicated.
: CREATE UNIQUE INDEX  "index_tags_on_name" ON "tags"  ("name")
```

This migration fails if there are duplicates:
https://github.com/mbleigh/acts-as-taggable-on/blob/v3.5.0/db/migrate/2_add_missing_unique_indices.rb#L3

See merge request !623

Change the MySQL collation type to utf8_bin to ensure case-sensitive tags are supported

MySQL tables had a collation type of utf8_ci, which makes the name case-insensitive. Previously this migration was run only after indices were built, but it needs to happen before if there is existing data that is case-sensitive. This is an idempotent change, so applying it again in the existing migration (20150425164651) should not change anything.

This is related to !623, but this migration should run before that one.

* Closes #1589
* Closes https://github.com/gitlabhq/gitlabhq/issues/9255

See merge request !625

Conditionally remove indices that may not exist in pre-GitLab v6.7 installations

In pre-GitLab v6.7 installations, the `tagging` indices were directly added to the init schema and not added in a migration. Anyone using this older DB schema may not have these indicies, so only remove them if they are there.

Closes #1593

See merge request !626

Closes #1593

new issue after release

See merge request !1810

Fix tooltips for event filter links

See merge request !618

Improve handling of large diffs

Diffs with a large number of changed lines time out (504 HTTP error) or
generate a HTML page that's so heavy web browsers struggle with it.

https://github.com/gitlabhq/gitlabhq/pull/5014 introduced limits on
commit line count so that only a safe portion is rendered. This was
later undone by code refactoring in be5b6db8, e0eb4803 and c741fcab.
This patch re-introduces a safe limit on number of lines.

See merge request !539

Fix bootlint issues on git clone holder bar

It looks like SIGTERM may not be enough to shut down a Sidekiq process
when its RSS has gotten too big. This change will allow us to experiment
with sending SIGKILL instead of SIGTERM to Sidekiq processes on
gitlab.com.

Add yanking pkg example to the release documentation.

See merge request !1809

Make the first branch pushed to an empty repository the default HEAD

In an empty repository, pushing a new branch not called "master" would leave HEAD in an unknown state, causing ambiguity if another branch were pushed. This could in turn cause a new protected branch to be created and cause the default branch to change.

* Closes #1561
* Closes #1576
* Closes https://github.com/gitlabhq/gitlabhq/issues/8883

See merge request !614

revert bad link in documentation

See merge request !1808

Added link reset root password

Added text to link "How to reset root password"

cc/ @sytse

See merge request !1799