Add 'Close' button for bugs

This commit introduces a new 'Close' button in the View Issue Details
page, making it easier and more intuitive for reporters to close their
own issues (when $g_allow_reporter_close = ON).

It also improves the access level verifications, simplifies the code and
makes it more readable by using standard access check functions.

This is an improvement over the functionality implemented in 0011502 not
only in terms of usability, but also for security as in some specific
cases users could actually bypass the workflow.

Fixes 0014156