Make project key regexp editable through web interface

Currently the regular expression defining valid project keys is defined in WEB-INF/classes/jira-application.properties:

jira.projectkey.pattern = ([A-Z][A-Z]+)

Since:

• quite a few people want to edit this
• once edited, data won't work properly in JIRA instances with the default regexp

we should store this regexp in the database, and make it editable through the web interface.