docs(i18n): internationalized the documentation

• Updated the Makefile to generate the templates in the locale folder.
• Added necessary fields to conf.py.
• Updated the Transifex configuration file to cover the documentation.
• Added *.mo and .po to .gitignore.
• Updated the documentation about writing documentation and releases.

Fixes #5899
chore(i18n): update translations
chore(release): v1.9.0-rc.3
Merge pull request #6973 from Srokap/release-1.9.0-rc.3

Release 1.9.0 rc.3
Merge pull request #6952 from Gallaecio/doc-i18n

docs(i18n): Documentation internationalization — The basics
fix(discussion): no longer show entity menu items on non-discussions

A bug in the discussion menu hook could cause discussion edit
links to show on other entities like users.

Fixes #6508
Merge pull request #6977 from jdalsem/discussion_entity_menu

fix(discussion): no longer show entity menu items on non-discussions
chore(vendors): Include PHPUnit (just like SimpleTest)
chore(tests): Make commit message test names into full sentences

This makes the intent of the tests more obvious.
chore(tests): Turn off Travis commit message tests

These have proven to be very extremely flaky since they are tied to
particular SHAs which may or may not be available in the git history
depending on how deep of a checkout was made.

We want unit tests to pass or fail depending on the current
state of the project, not the entire history of it.
Merge pull request #6986 from ewinslow/fix-commit-tests

Fix commit tests
fix(router): Can return 'handler' param in `'route', $identifier` hook again

In 1.8, it was possible to delegate to a different page handler by registering
a callback for the `'route', $identifier` plugin hook that looks like so:

```php
$result['handler'] = 'other';
return $result;
```

Then during 1.9 it was changed to expect the key to be 'identifier':

```php
$result['identifier'] = 'other';
return $result;
```

This change makes it so that both approaches will work as intended.

Fixes #6696