Fix error when migrating with existing `webhooks.tag_id` foreign

[dsevillamartin]

Fixes #64

Changes proposed in this pull request:

• Create migration that removes both foreign key & index of tag_id column

Reviewers should focus on: I think this should work with prefixes... but not entirely sure. Haven't tested that aspect.

Confirmed

• Frontend changes: tested on a local Flarum installation.
• [X] Backend changes: tests are green (run composer test).

[jtojnar]

Can confirm the migration succeeded with 166861ed0d659eed5c4e611347d0233b7f55894a:

Migrating extension: fof-webhooks
Migrated: 2023_06_07_000000_remove_tag_id_constraint
Migrated: 2023_06_07_change_tag_id_to_json

[dsevillamartin]

Thanks for verifying @jtojnar ! I've released v1.2.2 with this fix.