Replace tera with askama

[GuillaumeGomez]

This is the WIP (doesn't compile yet!) for the tera to askama switch. I need to change/fix some things in askama first:

• block and endblock should be allowed to have a name (it's pretty nice for long blocks).
• There is currently no way to know if a variable is defined or not (not a blocker but it would be nice to have).
• extends/import seem to be buggy, I need to investigate what's going on.
• I'd like if macros could have named arguments.
  • As a "bonus", it'd be nice if filters could have named arguments but not sure if it's possible.