Project creation: custom message for users without enough rights

This allows instance Administrators to customize the message displayed
to users that don't have any creation rights in place of the 'New Project'
page.

The message is rendered with Markdown, for easy links to documentation etc.
For instance, on foss.heptapod.net, it could state that project creation
is upon approval, and link to the `Hosting Request` issue creation form.

--HG--
branch : heptapod-stable
12 jobs for !280 with topic/heptapod-stable/user-cannot-create-project in 14 minutes and 40 seconds (queued for 6 seconds)
latest detached
Status Name Job ID Coverage
  Lint
passed rubocop #247055

00:04:10

 
  Test
passed rspec-regular 1/8 #247059

00:06:45

passed rspec-regular 2/8 #247060

00:08:44

passed rspec-regular 3/8 #247061

00:07:00

passed rspec-regular 4/8 #247062

00:07:55

passed rspec-regular 5/8 #247063

00:10:26

passed rspec-regular 6/8 #247064

00:08:23

passed rspec-regular 7/8 #247065

00:09:25

passed rspec-regular 8/8 #247066

00:08:53

passed workhorse:test using go 1.15 #247057

00:01:37

passed workhorse:test using go 1.16 #247058

00:01:38

passed workhorse:verify #247056

00:01:39