Mattermost-server: Write an automated test using `cypress` for "Theme Colors - Color Picker"

[esethna]

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Notes: Jira ticket

Test Key: AS14318

1. Go to Account Settings -> Display -> Theme
2. Select "Custom Theme"
3. Use one of the color pickers to edit the theme
4. Save

Expected:
Custom theme colors are saved.

See our end-to-end testing documentation for reference - https://developers.mattermost.com/contribute/webapp/end-to-end-tests/

[m3phistopheles]

claiming this one!

[saturninoabril]

Sure, thank you @m3phistopheles!

[a-arias]

I'm going to take this ticket 💃