Mentions does not respect plain-text renderer or {noformat} blocks

When a field uses the plain-text renderer, or part of a field is inside a {noformat} block, we should not parse that text for mentions.

Reasons:

• I don't think it's right to have a field type that has historically been pure, plain text; to now respond to one single markup tag in one specific way.
• In cases where plain-text has been explicitly enabled (that is, in behind-the-firewall instances, or where {noformat} is used), it's because the admin specifically wants that value to be treated as plain-text - e.g. stack traces or logs. If we no longer respect that, eventually we'll get customers asking for a plain-plain text field.
• Plain-text is also used for performance reasons that may be impacted by this. If large amounts of text are accidentally inserted into a wiki-rendered JIRA field, the issue becomes unviewable and uneditable. In these cases, Support switches the field renderer to plain-text to fix it up. According to Studio support, this happens roughly once a month on hosted. We need to be sure that Mentions' addition of rendering to this field type does not impact this use case.
• The current behaviour is not that great - if you mention someone in a plain-text field, you get the raw [~pwyatt] markup appearing on the View Issue screen (which means nothing to end-users on a plain-text system) and the whole field is rendered as wiki markup in the Mentions email (which is incorrect).
• In Confluence, text inside {code} blocks is not parsed for Mentions.

That said, disabling Mentions in plain-text mode would essentially turn off this feature for most OnDemand customers, where the default comment/description renderer is plain-text. Obviously that's not desirable.
Some solutions:

• In the Mentions advertising, link to the instructions for enabling wiki markup.
• Send it out dark in 5.0 OnDemand for plain-text customers, and in the next OnDemand release, add a one-click feature for admins to convert all their comment/description fields to wiki renderer. This would get around the pain that is the Field Configuration administration pages.