Unify terminology to use base file instead of template

Template is too confusing for gettext users.
Remove not needed code
Explicitely test creating of po
Fix condition for base file validation
Test for finding source string as well
Refuse to load .pot as base file

Previous using of template in admin interface lead many users to
specifying pot (PO template) as it, what is wrong as this (now called
base file) is meant only for monolingual formats.

There don't seems to be any legitimate use of pot file as base file, so
let's completely forbid this option.