Follow Me: add a button-only mode

[mattwiebe]

At long last, you can configure the "Follow Me" button to have a button-only mode.

Fixes #478

Proposed changes:

• Add a "Button Only Mode" toggle to the block
• Make button text configurable
• Add a "Button size" option (follows the Button component's sizes: "small" | "default" | "compact"

The sidebar looks like this now:

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

Before trying this PR, configure various instances of the Follow Me block. Ensure that they render the same way after this branch is run.

After checking this branch out, try out the new functionality and ensure everything behaves properly.

[obenland]

Upgrading worked well with the few instances I created ahead of time, nice!

There are some polish opportunities with the button-only mode:

• With color customizations there are some unexpected results, like the text color not being customizable and text/background color being the inverse.
• The button maintains margins around it, that make it behave and look different from the Core button block/

Is there a way to re-use the Core button and inherit its customizations when in button-only mode?