Draft: update to upstream 2.5.0

this has several problems still:

• docs are completely gone, we'd need to package Antora to get the HTML docs back. for now i threw them in .docs as plain text, probably pretty badly
• i haven't taken over the package, and therefore https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1003031 is still open

what i did here is try to follow the dgit-maint-debrebase manpage, so i did the following steps:

1. clone this repository: debcheckout elpa-projectile
2. add the upstream remote: git remote add upstream https://github.com/bbatsov/projectile.git
3. git fetch --tags upstream, which brought in new upstream tags that seem to be missing from this repo
4. git debrebase new-upstream 2.5.0, which created a ton of conflicts
5. painstakingly resolve the conflicts

Most of the conflicts where us obstinately trying to add apt install recommendations everywhere instead of just trusting users can figure out on their own which packages are in Debian and which are not. I think that's really a lot of work for not much benefit, personally, but I did the merge anyways. I haven't looked to see if there are more packages that could be added in that stack.

There's otherwise a lot of commits here because upstream moved quite a bit since we last updated this package, which makes this MR additionnally harder to read. So i guess you have to trust me on this?

I haven't actually audited the upstream code or tried to build this package.