[NEW] Hide inactive rooms

[mrsimpson]

What this adds

As time does by, users are being subscribed to many rooms. The left-hand side navigation gets messy. This PR adds an action for the user to hide inactive rooms.

Note the read conversations vanishing ;)
Remark: the icon is eye-off - it's just not shown on my local dev-env

How this is implemented

• It adds an option to the sort-list. There is no better place for cross-room-actions, imho - the user preferences would be too far off.
• With a modal, a user can define how old the minimum age is for rooms to be hidden.
• Favorites and unread rooms are exempted
• The implementation is similar to the normal hide-leave-actions with respect to package and model assignment.

What could be improved

• Default value for the modal. This needs a modification of the modal (modal.html, line 36ff ({{#if input}})). I didn't do this since it's a reuse component and wanted to get your feedback first.
• Create a regular ob and have the age defined by each user in its preferences (might be a second PR)

[kaiiiiiiiii]

IMO the user preferences are the perfect place for such settings. It’s something you may change once in a while, but nothing that needs to be super accessible :)

Awesome PR btw., I really missed that feature after we migrated from slack (:

[graywolf336]

I like the way this one is set up, as the user preferences feels hidden.

[mrsimpson]

@ggazzo Nobody cared about this change so far - and I think it should be implemented properly. Can you or someone from @RocketChat/core please give some feedback on whether this feature is desired?

[mrsimpson]

Closing this MR since I'll not be adapting it for a merge. However, I consider the feature elementary, but with another implementation