customcommands: thread editing functions

[SoggySaussages]

Continue forum and thread support for custom commands. This PR focusses on editing existing threads, closing, locking, opening again, slow mode, auto-archive, invitability, and forum pinning and editing tags.

New functions

Function	Description
closeThread thread (flag)	Closes the specified thread, or the current thread if set to nil. flag part is a bool and if set as true (false is optional) locks the thread as well as closing it.
editThread thread (values)	Edits the specified thread, or the current thread if set to nil. values must be key-value pairs of which parameters to edit. Available values:slowmode - provide length in seconds of slowmode; auto_archive_duration - provide duration in hours, must be either 60, 1440, 4320, or 10080; invitable - boolean, set to true to allow non-moderators to add members to the thread (only available for private threads) tags - either name of one tag, or slice of tag names. Overwrites the current tags (only available for forum posts).
openThread thread	Opens and unlocks the specified thread, or the current thread if set to nil.
pinForumPost thread	Pins the specified thread, or the current thread if set to nil in the parent forum channel.
unpinForumPost thread	Unpins the specified thread, or the current thread if set to nil from the parent forum channel.

I recommend reviewing first commit for simplest diff, second commit exclusively re-orders and third commit is minor bug fix.