Various syntax fixes in INSTALL.rst
exchange "kotti" by "external" (see #836)
allow instance-sepcific staticpages api address
link to staticpages inside instance

if instance specific staticpages are enabled.
Affects only external navigation and entity_url.
help_link inside instance if instance-specific staticpages
delete proposal description only if exists
category links in sidebars
Merge pull request #846 from xi/feature-fix-proposal-delete

delete proposal description only if exists
Merge pull request #847 from xi/feature-category-link

category links in sidebars
Fix category url for global categories
Merge pull request #843 from rowanthorpe/patch-1

Various syntax fixes in INSTALL.rst
fix remaining static links
Merge pull request #845 from xi/feature-instance-staticpages

Allwo to set staticpages backend for instances
i18n
fixup ret_url to came_from
Merge pull request #850 from xi/feature-category-link-url

Fix category url for global categories
catch external nav connection error
catch external nav JSON error
further improving external staticpages error handling