Adds a 'sticky' column to threads and rewrites the thread index to
take sticky markings into account when ordering threads.
Stickies are not yet highlighted in any way in the forum overview.
This is a simplification over the previous approach. The OP of a
thread is just a normal post like any other in this model, which
allows some code simplifications (and future query convenience).
Sets up the structure required to carry Tera templates in the
actix-web state.
The (still very barebones) index renderer has been updated to render a
Tera template.
