refactoring: Remove `version` in favor of `tag_name`

[Septias]

Change the version field to tage_name. While doing so I also refactored some of the code, mainly:

• Removed the subimmer_uri field aswell
• Remove logic for multiple frontend xdcs
• Merged two migrations

closes #184

[hpk42]

tested the branch (but only quickly) and it seems to work nicely. a bit unsure about adding "Tag: ...." as a new metadata line, how about "Tag: v2.0.2 from 7/19/2023" instead of Date and Tag lines? cc @r10s

[r10s]

a bit unsure about adding "Tag: ...." as a new metadata line, how about "Tag: v2.0.2 from 7/19/2023" instead of Date and Tag lines? cc @r10s

i understand that we want to be more precise in the data structures, however, for the average user, i think "Tag" is too technical in the UI, "Version" would be much better, ppl understand what that is.

if we do not want the word "Version" or "Release" in the UI ("Version v2.0.2 from 7/19/2023" or "Release v2.0.2 from 7/19/2023"), maybe just append the tag in brackets to the date line?

Date: 7/19/2023 (v2.0.2)

without much explaining what it is exactly - end user will see it as a version, devs can know it is just a tag.

in general, however, i think it is a good idea to have tag and date in the same line, they belong strongly together