Add templates for issues and pull requests

GitHub recently introduced templates for issues and PRs.
These will hopefully improve both.

//FREEBIE

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,20 @@
+<!-- You can remove this section if you have contributed before -->
+### First time contributor checklist
+<!-- mark with x between the brackets -->
+- [ ] I have read the [Contributor Guidelines](https://github.com/WhisperSystems/Signal-Desktop/blob/master/CONTRIBUTING.md)
+- [ ] I have signed the [Contributor Licence Agreement](https://whispersystems.org/cla/)
+
+### Contributor checklist
+<!-- mark with x between the brackets -->
+- [ ] My changes are rebased on the latest master branch
+- [ ] My contribution is fully baked and is ready to be merged as is
+- [ ] I have run the tests on these platforms:
+ * GNU Hurd 1.0, Chrom{e,ium} X.Y.Z
+ * Amiga OS 3.1, Chrom{e,ium} Z.Y
+- [ ] My commits are in nice logical chunks
+- [ ] I followed the [best practices](http://chris.beams.io/posts/git-commit/) when writing my commit messages
+- [ ] I have made the choice whether I want the Bithub reward or not by omitting or adding the word `FREEBIE` in my commit message
+
+----------
+
+### Description