Validate icon existence when setting project icon
Summary:
Do not allow setting an invalid project icon via the `project.edit` Conduit API but validate the value.
Same game as in D26430.
Closes T16322
Test Plan:
* Run `echo '{"transactions":[{"type":"name","value":"projectWithInvalidIcon"},{"type":"icon", "value":"noexxxist"}]}' | /var/www/html/phorge/arcanist/bin/arc call-conduit --conduit-uri http://phorge.localhost --conduit-token "cli-xxx" project.edit --`
* Succeed before applying this patch
* Fail after applying this patch:
```
{
"error": "ERR-CONDUIT-CORE",
"errorMessage": "ERR-CONDUIT-CORE: <project.edit> Validation errors:\n - Value for \"project:icon\" is invalid: \"noexxxist\".",
"response": null
}
```
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16322
Differential Revision: https://we.phorge.it/D26459