* Minor UI fixes

* PLT-3921 System Console Recent Active Users

* PLT-4061 - Updating pdf preview pages message

* PLT-4063 - Fixing spacing for LDAP button

* PLT-3837 - Showing reply button on mobile in the RHS

* PLT-4046 - Cantering tutorial on older mobile versions

Does this by adding special case regexes to the @mention importing code
in the Slack importer for these three special mention types.

Fixes PLT-4053

Update text for restricting integration management to admins

* Add Heroku deploy button

* Proposed README tweaks (#3867)

* Proposed README tweaks

* Update README.md

Change "Native Apps" to "Mattermost Apps" for consistency

* Update error page to use Markdown and add error for missing OAuth code

* Add line breaks to error messages 

This is to make the error message easier to read.

* Update en.json

* Moved {createSetHeaderButton(channel)} order

* Update channel_intro_messages.jsx

* Update user_settings_notifications.jsx

* Update en.json

This allows mattermost to use a non-tls connection with a SMTP server that
supports PLAIN auth (but not LOGIN). The go library explicitly allows PLAIN
auth over non-tls connections - https://golang.org/src/net/smtp/auth.go#L55

Fixes #2929

* Update custom_integrations_settings.jsx

* Update en.json

* Update custom_integrations_settings.jsx