Fix labels on Github issue templates. (celery#5955)

Use quotation marks to escape labels on Github issue templates. This
prevents the colon from breaking the template.

Author: martey

.github/ISSUE_TEMPLATE/Bug-Report.md

@@ -1,7 +1,7 @@
 ---
 name: Bug Report
 about: Is something wrong with Celery?
-labels: Issue Type: Bug Report
+labels: "Issue Type: Bug Report"
 ---
 <!--
 Please fill this template entirely and do not erase parts of it.

.github/ISSUE_TEMPLATE/Documentation-Bug-Report.md

@@ -1,7 +1,7 @@
 ---
 name: Documentation Bug Report
 about: Is something wrong with our documentation?
-labels: Issue Type: Bug Report, Category: Documentation
+labels: "Issue Type: Bug Report, Category: Documentation"
 ---
 <!--
 Please fill this template entirely and do not erase parts of it.

.github/ISSUE_TEMPLATE/Enhancement.md

@@ -1,7 +1,7 @@
 ---
 name: Enhancement
 about: Do you want to improve an existing feature?
-labels: Issue Type: Enhancement
+labels: "Issue Type: Enhancement"
 ---
 <!--
 Please fill this template entirely and do not erase parts of it.

.github/ISSUE_TEMPLATE/Feature-Request.md

@@ -1,7 +1,7 @@
 ---
 name: Feature Request
 about: Do you need a new feature?
-labels: Issue Type: Feature Request
+labels: "Issue Type: Feature Request"
 ---
 <!--
 Please fill this template entirely and do not erase parts of it.