Improve the UX of the built-in bridges dialog

We've observed that users often find it difficult to differentiate between Tor Browser's various bridge options, tend to choose a built-in bridge option at random (see: tpo/ux/research#100 (closed)), and have trouble figuring out how to connect afterwards (see: #41060 (closed)).

In response, we're proposing:

• Updating the “Bridges” section description on about:preferences#connection to include the word “securely”.
• Updating the title and description of the built-in bridges dialog
• Updating the individual descriptions that accompany each of the built-in bridge options
• Fixing the size and styling of the dialog and its constituent elements to make it more consistent with Firefox
• Replacing the OK button with a Connect button when not connected
• Adding a ✔ Connected flag to indicate with built-in bridge option Tor Browser is currently using

The Figma file is ready for dev handoff here: Figma link

These fixes will be ran through additional usability testing in March/April as part of Sponsor 30 before they reach stable in 12.5.