Fix heatmap activity (#15252)

* Group heatmap actions by 15 minute intervals

Signed-off-by: Sidd Weiker <siddweiker@gmail.com>

* Add multi-contribution test for user heatmap

Signed-off-by: Sidd Weiker <siddweiker@gmail.com>

* Add timezone aware summation for activity heatmap

Signed-off-by: Sidd Weiker <siddweiker@gmail.com>

* Fix api user heatmap test

Signed-off-by: Sidd Weiker <siddweiker@gmail.com>

* Update variable declaration style

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

models/fixtures/action.yml

@@ -32,3 +32,27 @@
   repo_id: 22
   is_private: true
   created_unix: 1603267920
+
+- id: 5
+  user_id: 10
+  op_type: 1 # create repo
+  act_user_id: 10
+  repo_id: 6
+  is_private: true
+  created_unix: 1603010100
+
+- id: 6
+  user_id: 10
+  op_type: 1 # create repo
+  act_user_id: 10
+  repo_id: 7
+  is_private: true
+  created_unix: 1603011300
+
+- id: 7
+  user_id: 10
+  op_type: 1 # create repo
+  act_user_id: 10
+  repo_id: 8
+  is_private: false
+  created_unix: 1603011540 # grouped with id:7