[ELI5] Validate too long handles in signup

[mozzius]

Checks if a handle is too long in the signup flow. Handles can be up to 253 chars normally, but for PDS service handles the max is 30, including domain. I added an additional flag to the validateHandle function to lower the length check appropriately. I made it generic for other PDSes, rather than hard-coding 18 chars. I also corrected the logic in the change handle screen since that was hardcoded to 18.

Test plan

1. Confirm that the limit really is 30 chars - go through the signup process on main with a 19 char username and confirm it fails
2. Check that the PR change catches it, and check that the error message is correct

[github-actions]

Old size	New size	Diff
6.86 MB	6.86 MB	-50 B (-0.00%)

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[estrattonbailey]

Yep nice one!