Improve explanations in ACP extensions page and remove HTML

Some of the explanations do not work very well with how they're currently being used. As described by Matt on GitHub:

Small issue. The EXTENSION_***_EXPLAIN language keys being used as href titles isn't working because they contain HTML:

Composer based extension "Update" should say:

Updating an extension will disable the currently installed extension and delete the extension’s files from the filesystem. The extension's most recent compatible release will be uploaded to your board and re-enabled.

Composer based extension "Remove" should say:

Removing an extension will remove all of its data and settings and delete the extension’s files from the filesystem.

In general, we should aim at removing the HTML from these help lines to prevent similar issues from occuring.