[github] Issue templates: populate some labels

cc @bramvanroy @stefan-it

.github/ISSUE_TEMPLATE/--new-model-addition.md

@@ -2,7 +2,7 @@
 name: "\U0001F31F New model addition"
 about: Submit a proposal/request to implement a new Transformer-based model
 title: ''
-labels: ''
+labels: New model
 assignees: ''
 
 ---

.github/ISSUE_TEMPLATE/migration.md

 ---
 name: "\U0001F4DA Migration from pytorch-pretrained-bert or pytorch-transformers"
-about: Report a problem when migrating from pytorch-pretrained-bert or pytorch-transformers to transformers
+about: Report a problem when migrating from pytorch-pretrained-bert or pytorch-transformers
+  to transformers
 title: ''
-labels: ''
+labels: Migration
 assignees: ''
 
 ---

.github/ISSUE_TEMPLATE/question-help.md

@@ -26,4 +26,4 @@ assignees: ''
 
 <!-- You should first ask your question on SO, and only if
      you didn't get an answer ask it here on GitHub. -->
-**A link to original question on Stack Overflow**: 
\ No newline at end of file
+**A link to original question on Stack Overflow**: