Purpose

This plugin has three related functions, each optional of course:

• A link to mark threads as solved/unsolved, which adds/removes a prefix.
• Optionally a banner on solved threads, telling the user more visibly that the thread is already solved.
• Optionally a banner on old unsolved threads that the thread is abandoned (age based on last post).

Features

• Once solved, the link changes its text and allows to revert the solved status again.
• Solving is possible without edit permissions, eg. when there is a time limit for them.
• 4 new permissions: Solve/Unsolve own/other threads. Alternatively, ManageAnyThreads includes them too (because it allows to modify the thread prefix freely).
• Link and/or banners can be above the first post, and/or below the last one, right above the editor.
• How old a threads should be to show the old banner, if activated, can be configured (days).
• Both kinds of banners are only visible if the viewing user is able to post in the thread
• Both kinds of banners have optional settings to display them only if the viewing user has less than x posts
  (as in, experienced users don't need reminders)
• By configuring the prefix only for certain forum nodes, the solve links (and permissions) exist only in these nodes too
• It can be configured if the old banner should be active in prefix-less forums too.
• Templates and phrases start with sheel_solved
• No database changes
• No cost, no branding, MIT license

Installation

1. Create a prefix which should be used for the solved threads, ie. text, color, and allowed nodes. (Note that ✔from the screenshots below is not an image, it's a Unicode character. If you want to use it as prefix, just copy-paste it.)
2. Upload the "Sheel" folder to library/ of you Xenforo installation and install the XML file in the ACP.
3. Go to the option page and set (at least) the prefix id there.
4. Set user group permissions.

Done.
If you like to, modify phrases and/or templates too (the names all start with sheel_solved).

Screenshots:

Example how the solve link above the posts could look like (anonymized):

Link below the posts:

A possible prefix:

A different prefix configured:

Optional solved-banner, in this case below the posts:

Optional banner for old unsolved threads:

Options page: