FIX Only block root vendor folder

Use RewriteRule instead to take in account any subfolder via RewriteBase. Deny ss-cache and composer via RewriteRule too.
BUG GridFieldFilterHeader works without non-filterable cols

Previously relied on the presence of a last column which wasn't filterable,
commonly a GridFieldEditButton. If this wasn't present, the filter buttons
were never added, leading to the GridField JS reload request being sent
without the required button form action, so GridFieldFilterHeader->handleAction()
was never called.
Update server-requirements.md
Update common-problems.md
Update common-problems.md
Update common-problems.md
Merge pull request #1982 from chillu/pulls/gridfield-filter-cols

BUG GridFieldFilterHeader works without non-filterable cols
Merge pull request #1786 from colymba/3.0-htaccess-fix

vendor folder is blocked only if outside themes
Merge remote-tracking branch 'origin/3.0' into 3.1