* Allow customizing max form size from env vars
* Add error page for unprocessable entities
And change default http port to 7878
* Improve char counter: under the editor, more discrete, and give it a default value
All the template are now compiled at compile-time with the `ructe` crate.
I preferred to use it instead of askama because it allows more complex Rust expressions, where askama only supports a small subset of expressions and doesn't allow them everywhere (for instance, `{{ macro!() | filter }}` would result in a parsing error).
The diff is quite huge, but there is normally no changes in functionality.
Fixes#161 and unblocks #110 and #273
