Handle restricted filesystem permissions and allow for disabling block editor support

[mlwilkerson]

closes #242

1. Make it a warning, not a fatal error, when we can't fetch SVG support styles.
2. In the warning, show what's expected so users can possibly manually intervene.
3. Add a font_awesome_disable_block_editor_support filter to disable the block editor support entirely.
4. When checking for the presence of the SVG stylesheet, use filesystem access first. If that fails for lack of permissions, attempt to fetch it via wp_remote_head().
5. Change Classic Editor support code to be more compatible with other third-party plugins. See here and here.

Using the New Filter

Add the following to your themes functions.php or equivalent:

add_filter( 'font_awesome_disable_block_editor_support', '__return_true' );

TODO

• add test coverage for the new configuration and failure cases
• revisit whether we need the try/catch in activate()