MAAS-90 | Improve Feed model

[tuomas777]

Added fields name, url_or_path, imported_at, import_attempted_at and last_import_error_message to Feed model. Idea is that there is now a separate name for the feeds, and the status of the last import is visible in the admin UI.

While at it, did also some other tweaks to feed admin UI and added some statistics about the data of the feed. Those were added just because it was very easy, and can be removed if we don't want them there 😄

[codecov-commenter]

Codecov Report

Merging #38 (4db6e10) into main (8fa2b86) will decrease coverage by 1.06%. The diff coverage is 65.55%.

@@            Coverage Diff             @@
##             main      #38      +/-   ##
==========================================
- Coverage   89.17%   88.11%   -1.07%     
==========================================
  Files          45       45              
  Lines        1303     1380      +77     
  Branches       97      103       +6     
==========================================
+ Hits         1162     1216      +54     
- Misses        111      134      +23     
  Partials       30       30              

Impacted Files	Coverage Δ
gtfs/management/commands/import_gtfs_feed.py	38.09% <25.00%> (-21.91%)	:arrow_down:
gtfs/admin.py	64.00% <44.82%> (-12.09%)	:arrow_down:
gtfs/importers/gtfs_feed_updater.py	94.44% <89.47%> (+12.62%)	:arrow_up:
gtfs/models/feed.py	95.55% <93.75%> (+1.61%)	:arrow_up:
gtfs/importers/gtfs_feed_importer.py	90.09% <100.00%> (+0.22%)	:arrow_up:
bookings/api.py	100.00% <0.00%> (ø)
bookings/serializers.py	100.00% <0.00%> (ø)
bookings/models.py	94.91% <0.00%> (+0.08%)	:arrow_up:

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