Implement asset constraints in the user DB

[LefterisJP]

Fix #2906

Introduces a new assets identifier table which is a mirror of the globalDB assets. Only contains the identifier. All asset identifiers across the DB are now foreign keys to that table.

This way if we ever need to change an asset saved across the entire database all we need to do is a cascade update.

TODO: Add tests for such cascade updating. Probably as part of #2898

[codecov]

Codecov Report

Merging #2938 (5bda306) into develop (7ba4940) will decrease coverage by 0.17%.
The diff coverage is 92.02%.

@@             Coverage Diff             @@
##           develop    #2938      +/-   ##
===========================================
- Coverage    65.32%   65.14%   -0.18%     
===========================================
  Files          533      537       +4     
  Lines        30569    30994     +425     
  Branches      4409     4496      +87     
===========================================
+ Hits         19970    20192     +222     
- Misses        9572     9741     +169     
- Partials      1027     1061      +34     

Flag	Coverage Δ
backend	77.99% <92.02%> (+0.31%)	⬆️
frontend_integration	45.50% <ø> (-0.23%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
rotkehlchen/db/upgrades/v25_v26.py	88.88% <89.71%> (+1.65%)	⬆️
rotkehlchen/api/rest.py	79.80% <100.00%> (+0.21%)	⬆️
rotkehlchen/db/dbhandler.py	80.60% <100.00%> (+0.23%)	⬆️
rotkehlchen/db/schema.py	100.00% <100.00%> (ø)
rotkehlchen/db/upgrade_manager.py	97.89% <100.00%> (ø)
...rontend/app/src/views/settings/GeneralSettings.vue	36.29% <0.00%> (-30.14%)	⬇️
frontend/app/src/utils/log-level.ts	72.72% <0.00%> (-27.28%)	⬇️
...c/components/status/notifications/Notification.vue	45.83% <0.00%> (-16.67%)	⬇️
frontend/app/src/store/notifications/actions.ts	48.78% <0.00%> (-12.20%)	⬇️
frontend/app/src/electron-interop.ts	26.53% <0.00%> (-6.81%)	⬇️
... and 18 more

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