[DB] Increase max length of link fields

The current core supports links with the maximum length of 256 chars in
header links of tt_content elements and links in table
sys_file_reference.

External links can have much more than 256 chars.

Increasing the maximum length to 1024 should be enough - even current
browsers are supporting much more than 1024 chars.

Resolves: #61944
Releases: master, 6.2
Change-Id: I2031ebef959de696bc320b927ec1a462bcd5dd72
Reviewed-on: http://review.typo3.org/33052
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>