validate length of external_feed url in the app
validate length of external_feed url in the app

database column is capped at 255, so validate that length in-app as well

fixes CNVS-8329

test plan:
- try to add an external feed with a url > 255 characters
- it should fail with an error messages that the url is too long

Change-Id: I17b1adc50073fb2598a025a85a87abefbc501bd4
Reviewed-on: https://gerrit.instructure.com/46646
Tested-by: Jenkins
Reviewed-by: Mike Nomitch <74443f6f110b6b7b934c89126258ed3d843d673f@instructure.com>
QA-Review: Adam Stone <a0b2b61e2edcf99aa16080c2254a6ae6923d1fac@instructure.com>
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

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