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
API UploadField functions on new records

Fixed regression from 1e5d40474d43ddb0ade22bbf7db5ec494b979fea (UploadField::canPreviewFolder).
Merged in pull request #2009 - (6018bdd631ec98345bba5cd26ef5a29f6f412678).
Merged pull request #1259 (34bfc862eee061ce30998a085aac6170022c3a7e).
Fixed scrutiniser issues
BUG Fixed major issue with testing dataobjects that implement TestOnly and extend non-TestOnly dataobjects. Database regeneration would incorrectly populate the ClassName column
BUG: correct handling of multiple classes
Merge pull request #2023 from g4b0/left-and-main-classes

BUG: correct handling of multiple classes
Merge branch '3.1-uploadfield-enhancement' of git://github.com/tractorcow/sapphire into tractorcow-3.1-uploadfield-enhancement
Fixed UploadField width (same as textual fields)

Was getting too wide due to padding which isn't applied
to other fields (e.g. TextField)
Merge pull request #2017 from tractorcow/3.1-classname-test-fix

BUG Fixed major issue with testing dataobjects that implement TestOnly
New video formats in allowed_extensions

See https://github.com/silverstripe/silverstripe-installer/pull/41
Improve docs (mostly spelling)
FIX: Add missing 'groupby' and remove redundant conditional in 'having'
Merge pull request #2029 from nedmas/patch-2

FIX: Add missing 'groupby' method