Apply Alpha and Nightly branding to unstable release channels

Right now, the only difference between each release channel is the color of the application icon. We should also update:

• The application name (i.e. "Mullvad Browser Alpha" and "Mullvad Browser Nightly").
• The start page in Alpha and Nightly.
• The about window.

Design todos

• Supply SVG assets for the Mullvad Browser app icon (stable, alpha & nightly).
• Create updated wordmarks incorporating the words "Alpha" and "Nightly", also in SVG. Note: each of these assets need to be the same size for Fingerprinting reasons.
• Optionally: adapt the link colors per release channel, testing for accessibility (AA compliance with WCAG 2.2). Note: the current yellow colors are derived from Fx's Acorn style guide, using different shades for default, hover and pressed states.
• Optionally: Create individual backgrounds per release channel. However shifting from a radial to a linear gradient may be necessary to create more predictable backgrounds for accessibility.
• Prepare designs for handoff in the 14.5 working file: Figma link
• Package up all assets produced for this task, and upload them to the Nextcloud folder: https://nc.torproject.net/s/odTMrPstNYnrsrE