Moved the string for the CSS style to display links as buttons in emails to a separate class, so that it can be reused in other mailers.

The new class is under /lib. I'd prefer it to be a rails helper, but that would require changing some Devise mailers (not the views) to include the helper, and I'm not a fan of the idea of customizing Devise mailers just for this.
Styled action link in "accept invitation" email as a bootstrap-like button.

Also the text in the confirmation and accept-invitation emails directs the user to "click this button", not to "follow this link" or "click on this link".
Style action link in reset-password email as a bootstrap-like button.

Also the text instructs the user to "click on this button", not "click on this link".
Wrapped action button in invitation emails with h2 tags.

Now it's structured the same as other emails sent by this app.
Style the action button in the unlock account email as a bootstrap-like button.

Also the text directs the user to "click on this button", not "click on this link".
Minor change in text for the opml export failure email.