Remove custom `IntegerConverter`

[lukasjuhrich]

Werkzeug now supports signed values viasigned=True for its IntegerConverter, so the workaround introduced in ea9bbc2 is unnecessary.

Relevant lines:

sipa/sipa/base.py

Lines 19 to 27 in 1d5547b

	class IntegerConverter(BaseIntegerConverter):
	"""IntegerConverter supporting negative values
	This is a Modification of the standard IntegerConverter which does
	not support negative values. See the corresponding `werkzeug
	documentation
	<http://werkzeug.pocoo.org/docs/0.10/routing/#werkzeug.routing.IntegerConverter>`_.
	"""
	regex = r'-?\d+'

sipa/sipa/initialization.py

Line 57 in b8d1188

app.url_map.converters['int'] = IntegerConverter

[lukasjuhrich]

It has been introduced to support negative values in query parameters, more specifically, to make an URL like https://agdsn.de/sipa/news/?start=-2&end=-1 work.