Fix mysql migration

Fixes exception noted in https://gitlab.com/gitlab-org/gitlab-ce/issues/913

```
** Invoke db:migrate (first_time)
** Invoke environment (first_time)
** Execute environment
** Invoke db:load_config (first_time)
** Execute db:load_config
** Execute db:migrate
== 20141121161704 AddIdentityTable: migrating =================================
-- create_table(:identities)
   -> 0.1214s
-- add_index(:identities, :user_id)
   -> 0.1660s
-- execute("INSERT INTO identities (provider, extern_uid, user_id)\nSELECT provider, extern_uid, id FROM users\nWHERE provider IS NOT NULL\n")
   -> 0.0518s
-- remove_column(:users, :extern_uid)
rake aborted!
StandardError: An error has occurred, all later migrations canceled:

Mysql2::Error: Duplicate entry 'ldap' for key 'index_users_on_extern_uid_and_provider': ALTER TABLE `users` DROP `extern_uid`/home/git/gitlab/vendor/bundle/ruby/2.1.0/gems/activerecord-4.1.1/lib/active_record/connection_adapters/abstract_mysql_adapter.rb:301:in `query'
/home/git/gitlab/vendor/bundle/ruby/2.1.0/gems/activerecord-4.1.1/lib/active_record/connection_adapters/abstract_mysql_adapter.rb:301:in `block in execute'
```

See merge request !1344

Release announcement

@dzaporozhets wants a hashtag https://gitlab.slack.com/archives/general/p1419317692000272

See merge request !1340

Migrations separate

Sytse will tweet on merge.

See merge request !1343

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Install guide

Add libkrb5-dev to upgrader /cc @dzaporozhets @valery

See merge request !1341

Mention the team

See merge request !1339

Rename sanitize in check task

This is to prevent the name clash between `sanitize_helper.rb` method sanitize and the method used to check if env variable is set in rake task.

See merge request !1336

Differentiate system notes

Remove unused has_gitlab_shell3? method

Use blob local instead of instance.

Disable / hide MR edit blob button if cannot edit.

Update gemnasium-gitlab-service to version 0.2.3

Fix dev user seed: ID was used twice.

Add tests for tree edit routes

Reduce the size of Docker image

Update the Omnibus package in Dockerfile

transfer error handler

[BUGFIX] Invalid branch in comparison

Fix Rake tasks doc README: add top level h1

Add theme type css class

Add link to OmniAuth config sample for Omnibus

Added a link to the OmniAuth config sample for Omnibus installations.

See merge request !1335

Update Documentation for 7.6 release

Updated the update and install documentation for the 7.6 release.

See merge request !1334

and link to missing to features.md.