Flowauth serverpage validation

[BhavinPanch]

Closes #748

I have:

• Formatted any Python files with black
• Brought the branch up to date with master
• Added any relevant Github labels
• Added tests for any new additions
• Added or updated any relevant documentation
• Added an Architectural Decision Record (ADR), if appropriate
• Added an MPLv2 License Header if appropriate
• Updated the Changelog

Description

Added client-side validation and error message on new server page for admin users.

[codecov]

Codecov Report

Merging #749 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #749   +/-   ##
=======================================
  Coverage   93.16%   93.16%           
=======================================
  Files         130      130           
  Lines        6522     6522           
  Branches      691      691           
=======================================
  Hits         6076     6076           
  Misses        326      326           
  Partials      120      120

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a7d8dd9...2939066. Read the comment docs.

[greenape]

Great stuff :)

[maxalbert]

Very minor comment: I personally like to be explicit about the condition that triggers a conditional branch. In this case it's obvious that the else branch can only be triggered if the servername is too long, but it's easy to later add another condition in the if statement and forget to update the else branch, and then the error message displayed is inconsistent with the condition that triggered it.

How about turning this else branch into another else if?

Suggested change

	} else {
	} else if (servername.length > 120) {

Of course then we need to add a catch-all else branch which blows up if it's triggered (because it should never be reached). I'm not sure what a good way to "blow up" is in Javascript land, though, so maybe this isn't a feasible approach. ;)