Make JIRA mentions compatible with Office365/Outlook's @Mention feature

As of 2016, Microsoft Office365/Outlook supports a @Mention feature, which highlights/filters emails which explicitly call out the recipient. This feature is similar in purpose and intent to JIRA's Mention feature (available since 2013).

Thus, the features are equivalent, however, the syntax of Mentions in JIRA email notifications is different and incompatible:

• JIRA uses [~username] syntax in text entry, and @username in the user interface
• Office365/Outlook expects e.g. @lastname (plain text) and <a href="mailto:username@host">lastname</a> (HTML)

This email combination is likely to be common among enterprise customers, so interoperability should be valuable. I am aware of other requests for improving JIRA Mentions, such as JRASERVER-37777, JRASERVER-41364, but this specific request should integrate better and have more immediate benefits to a large user base.

Since non-Office365/Outlook email won't necessarily honor the same @Mention syntax, this proposal should be a configurable option.