Useful Twig functions

[Starpaul20]

We should consider adding some Twig functions for the following:

• [ ] Inline Errors
• [ ] Stuff that has been run through the parser
• [ ] codebuttons (the editor)
• [ ] smilieinserter
• [ ] build_profile_link
• [x] my_date
• [x] multipage
• [ ] captcha
• [x] my_number_format
• [ ] get_friendly_size
• [x] trans function HTML safe
• [x] replacement for $lang->parse()
• [ ] get_thread_link

[JordanMussi]

Also https://github.com/mybb/mybb/pull/2975#discussion_r160036829

I wonder if it's worth marking the trans function as HTML safe... There are quite a few translations that have HTML in them unfortunately.

[Shade-]

Would be nice to add get_thread_link too.

[WildcardSearch]

Marked $lang->parse() replacement marked.