The parse_integer function was previously made to reject negative values by
default in https://github.com/element-hq/synapse/pull/16920, but the
documentation stated otherwise. This fixes the documentation and also:
- Removes explicit negative=False parameters from call sites.
- Brings the negative default of parse_integer_from_args in alignment with
parse_integer.
During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
The `parse_enum` helper pulls an enum value from the query string
(by delegating down to the parse_string helper with values generated
from the enum).
This is used to pull out "f" and "b" in most places and then we thread
the resulting Direction enum throughout more code.
