Validate icon existence when setting project icon master
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