Improve handling of flash messages

[UlrichB22]

#2162 adds flash messages in case of unbalanced moinwiki tags. This will often lead to duplicate messages.

The module 'converters/html_in.py' uses a class NoDupsFlash() to avoid duplicate messages. There should be a common solution for all converters.

[gmilde]

The message duplication in "moin-in" is due to both:

1. repeated, identical messages are shown again with "moin-in" (but suppressed by NoDupsFlash in "html-in").

2. All flash messages are shown two times when switching from the "edit" view to "show" via the [OK] button (but only once in the [preview] as well as when switching from preview to "show" (issue Flash messages are shown twice. #2204).

Only problem 1 may be solved by moving the NoDupsFlash class definition to converters or converter._utils and using it in converters.moin_in.
However, in large documents, it would be better to report all parse errors together with the source line number.

Problem 2 may be an idicator that the conversion is run two times...

[gmilde]

For an example of an error message with line number and backlink see the rST help
http://localhost:5000/help-en/rst#Error_Handling