[issue2551262] Make mail gateway issue matching more fault tolerant

New submission from Heiko Stegmann:

Roundup's mail gateway allows to set add email content to an issue by specifying the issue designator in square 
brackets at the beginning of the email subject line. However, the syntax must exactly follow the scheme [issue<num>]. 
It must be placed at the beginning of the subject line, no blank spaces are allowed, only square brackets will work, 
and I think it is also case sensitive.

However, it frequently happens that users do not follow this syntax, mostly unintentionally. In these cases Roundup 
will not find the matching issue and create a new one instead. In our tracker that has hundreds of users, this leads 
to a large number of useless issues that must be sorted out at great expense.

My request is to make the mail gateway based issue matching more fault tolerant. It should not matter where in the 
subject line the issue designator is placed. Blank spaces inside the brackets and brackets other than square should be 
allowed. It should also be case insensitive.

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components: Mail interface
messages: 7724
nosy: Heiko, schlatterbeck
severity: major
status: new
title: Make mail gateway issue matching more fault tolerant
type: behavior
versions: 2.2.0

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Roundup tracker <[email protected]>
<https://issues.roundup-tracker.org/issue2551262>
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