Wrote another class for solution lists
Allow only editors to edit problems, solutions and tags
Cache filter results only when needed
Update submodules
Added an issue page

It's not yet linked from other pages.
TeX output for issues page

Generate TeX files with problems or solutions published in a specific
issue.
Implemented a placeholder syntax for remarks

If remarks contain a "~", they will be wrapped around the proposer list,
which is put in place of "~".
Use SQLite transactions to combine write operations
Allow filtering out published problems, filtering by issues obsolete
Added calendar linking to issue pages
Changed menu icon, hide tag editor for non-editors