* Added model objects for emoji

* Added database tables for emoji

* Added settings for custom emoji

* Added serverside APIs and unit tests for custom emoji

* Added additional validation to catch duplicate emoji names earlier on

* Added additional validation to prevent users from adding emoji as another user

Conflicts:
	webapp/components/create_comment.jsx

* Comment out max file size settting in system console

* Remove pending post after successful create post request

* PLT-2058 Debugging incoming web hook content

This change debugs contents of incoming webhooks using l4g.

The problem is that in order to debug the request body, it neads to be
read. And a Reader can only be read once. Hence, the body is only read
for Debugging if it is actually enabled. Furthermore, a new reader is
created from the content of the old reader in order for the rest of the
method to work as usual (with or without debugging).

The debug statement is wrapped in a closure, so that the content is
only copied if Debug is actually enabled.

It is not possible to return `(string, string)` from the closure to
`l4g.Debug()`. That is the reason the debugging is not done with `=%v`,
but the translations strings end with a space.

I tested the change with a `application/json` HTTP header as well as
`payload=`

The debug method is extracted into util/log.go in order to be re-usable
for debugging `io.Reader`

* Added a config flag to turn off incoming webhook debugging

Setting `EnableWebhookDebugging` to false in the `config.json` will
disable the printing of the content of incoming webhooks to the console

* Defaulting webhook debugging to true

* Added the setting of debugging incoming webhooks to the system console

* scrolling fixes:
- jumps on free scroll when somebody posts a new message (and old one disappears at top)
- jumps on closing RHS (no compensation of resize)
- scroll blinking on opening/closing RHS (delay before resize compensation)

* fix scroll slipping up on multiple open/close RHS in case when (this.refs.postlist.offsetHeight % Constants.SCROLL_PAGE_FRACTION === 0)

* Fixed a typo to an error message regarding editing an account field handled by a login provider. (PLT-3156)

* Added 'field_handled_externally' to the en language file.
Other languages are needed.

PLT-3203 Commenting on the RHS while in permalink view makes the message not show up in the RHS

* Add server restart note to LDAP sync setting

* Add server restart note to LDAP sync setting

* Fixing attachment in RHS root post for compact view

* Improving spacing for mentions

PLT-3203 Commenting on the RHS while in permalink view makes the message not show up in the RHS (#3282)

* Add "Database:" to Mattermost About Dialog

The property has been named SQLDriverName to make a difference with the
FileSettings DriverName.

* Add missing string for 'Database' in i18n file

* Added support for DMs

* Checked username beginning characters

* Added user setting to auto collapse image previews

* Added slash commands for collapsing/expanding image previews

* Added translation strings for collapse setting

* Add default props for preview collapse setting

* Added message history

* Minor logical changes

* Fixed indexes resetting

* Fixed double messages

* Fixed resetting main history when RHS opened