only mount the extensions vendor dir if it is writable
whitespace
is_writable returns false on non existent file, enclosed block handles failure correctly
check dir exists
honour branding name
try a different method to copy to directory
add a small handler class to the filesystem mounter to check directories exist before mounting
catch the possibility that we cant make a public url
properly namespace
if the config directory does not exist, try and create
add secondary extensions nav with link to configuration files
this was an incorrect test, the mounted path needs but the file manager was mounting a non-existent path
Merge pull request #1688 from rossriley/extensions-improvements-fixes

Tests work here and the changes are solid.

Fixes #1618