Update org URL scheme from <id-slug> to <slug>

[magnified103]

Description

Type of change: new feature

What

Update organization URL scheme from organization/<id-slug> to organization/<slug> and redirect old URLs to the corresponding new one.

Add validation rule to ensure organization slugs begin with a-zA-Z.

Why

Slugs have been unique since #332, so there's no need to append the ids as in the old format.

How Has This Been Tested?

Tested on local.

Checklist

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• I have explained the purpose of this PR.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the README/documentation
• Any dependent changes have been merged and published in downstream modules
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By submitting this pull request, I confirm that my contribution is made under the terms of the AGPL-3.0 License.

[hieplpvip]

You missed a few URLs but otherwise good work! LGTM