Allow mentions of groups

[julianlam]

As an update to mentions, be able to tag groups:

• Core Groups.search(), similar to User.search()
• Mentions Search for both users and groups in autocomplete
• Core Admin created groups should have a "slug" set, defaulted to a slugified version of the group name
• Core Notifications.pushToGroup() which would push a nid to that group's users
• Mentions Notify users in group

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Re: #1326 -- would be useful to tag mobile compatibility team in a single message.

This also goes halfway towards exposing the groups system to the front-end, so users can be added to special groupings and be shown on the board as a type of flair, e.g. "Translators", "Mobile Compatibility Team"

[julianlam]

Remaining work can be carried out in plugin itself. Closing.

[julianlam]

... didn't work

[barisusakli]

julianlam/nodebb-plugin-mentions#34

[julianlam]

Doesn't quite work (the group itself is not linkified in the post body)

[barisusakli]

Doesn't work if a group has a space in its name works for single word group names.

[a5mith]

Changing the URL of the group might affect this pull request. Depends if the url changes from name to slug or something like that. 👍

[andreapernici]

Give the possibility to disable this group mention.

[barisusakli]

julianlam/nodebb-plugin-mentions#42