Namespace housekeeping

Two changes:

  - Both `RedmineJIRAExporter` and `RedmineJiraExporter` were in use.
    Standardize on `RedmineJiraExporter`.

  - Shorten the names of the controller and the issue view hook.
Show banner on projects with jira_export enabled

The view hook no longer aborts if `jira_export` is enabled. However, tickets in
these projects will not be available for export. This is useful as the plugin
can be enabled for all projects to inform users that ticket tracking has moved.

Closes GH-6.
Provide instructions if an issue is not exportable

For issues that are closed, do not have a configured recieving JIRA project or
for which users don't have permission to export, add a note explaining that an
issue should be opened on JIRA and linked to the Redmine ticket.

Closes GH-3.