[5.x] Display custom logo as plain text

[daun]

Description

Add an option to display the site's name in plain text in the control panel, as suggested in feature idea #1185.

Advantages of doing that

• Consumes less bandwidth than a logo file
• Light/dark mode comes along for free
• Less setup for new projects: just paste in the name

Details

• When defined, logo image urls take precedence over logo text
• The logo text goes into a new statamic.cp.custom_logo_text config key
• Can also be set from a new STATAMIC_CUSTOM_LOGO_TEXT environment variable
• I guess this can be backported to 4.x without problems
• As far as I can tell, there's currently no tests for the custom logo behavior, so I haven't added any
• Manually tested, though :)

Screenshots

     

Closes statamic/ideas#1185