Ensure valid git author names passed in signatures (#5774)

* Ensure valid git author names passed in signatures

Fix #5772 - Git author names are not allowed to include `\n` `<` or `>` and
must not be empty. Ensure that the name passed in a signature is valid.

* Account for pathologically named external users

LDAP and the like usernames are not checked in the same way that users who signup are.
Therefore just ensure that user names are also git safe and if totally pathological -
Set them to "user-$UID"

* Add Tests and adjust test users

Make our testcases a little more pathological so that we be sure that integration
tests have a chance to spot these cases.

Signed-off-by: Andrew Thornton <art27@cantab.net>

models/fixtures/user.yml

@@ -19,7 +19,7 @@
   id: 2
   lower_name: user2
   name: user2
-  full_name: User Two
+  full_name: "   < U<se>r Tw<o > ><  "
   email: user2@example.com
   passwd: 7d93daa0d1e6f2305cc8fa496847d61dc7320bb16262f9c55dd753480207234cdd96a93194e408341971742f4701772a025a # password
   type: 0 # individual
@@ -37,7 +37,7 @@
   id: 3
   lower_name: user3
   name: user3
-  full_name: User Three
+  full_name: " <<<< >> >> > >> > >>> >> "
   email: user3@example.com
   passwd: 7d93daa0d1e6f2305cc8fa496847d61dc7320bb16262f9c55dd753480207234cdd96a93194e408341971742f4701772a025a # password
   type: 1 # organization
@@ -53,7 +53,7 @@
   id: 4
   lower_name: user4
   name: user4
-  full_name: User Four
+  full_name: "          "
   email: user4@example.com
   passwd: 7d93daa0d1e6f2305cc8fa496847d61dc7320bb16262f9c55dd753480207234cdd96a93194e408341971742f4701772a025a # password
   type: 0 # individual