Rename file as "mission"

The mission is the most important part. The values describes the
values with use in fulfilling this mission.

wiki/src/contribute/identity.html → wiki/src/contribute/mission.html

 [[!meta title="Mission & identity"]]
 
 [[!meta stylesheet="bootstrap.min" rel="stylesheet" title=""]]
-[[!meta stylesheet="identity" rel="stylesheet" title=""]]
+[[!meta stylesheet="mission" rel="stylesheet" title=""]]
 
 <div class="row">