Encourage +1's for prioritisation

Adds a small text to each issue and the PR template to encourage people to use
👍's for issues they're also interested in. See
https://github.com/NixOS/nix.dev/issues/359 for more information

.github/ISSUE_TEMPLATE/bug_report.md

@@ -38,3 +38,7 @@ Please run `nix-shell -p nix-info --run "nix-info -m"` and paste the result.
 [user@system:~]$ nix-shell -p nix-info --run "nix-info -m"
 output here
 ```
+
+### Prioritisation
+
+Add :+1: for [issues you find important](https://github.com/NixOS/nixpkgs/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).

.github/ISSUE_TEMPLATE/build_failure.md

@@ -32,3 +32,7 @@ Please run `nix-shell -p nix-info --run "nix-info -m"` and paste the result.
 [user@system:~]$ nix-shell -p nix-info --run "nix-info -m"
 output here
 ```
+
+### Prioritisation
+
+Add :+1: for [issues you find important](https://github.com/NixOS/nixpkgs/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).

.github/ISSUE_TEMPLATE/missing_documentation.md

@@ -30,3 +30,7 @@ assignees: ''
 
 <!-- propose a solution -->
 
+
+## Prioritisation
+
+Add :+1: for [issues you find important](https://github.com/NixOS/nixpkgs/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).

.github/ISSUE_TEMPLATE/out_of_date_package_report.md

@@ -46,3 +46,7 @@ maintainers:
 ###### Note for maintainers
 
 Please tag this issue in your PR.
+
+###### Prioritisation
+
+Add :+1: for [issues you find important](https://github.com/NixOS/nixpkgs/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).

.github/ISSUE_TEMPLATE/packaging_request.md

@@ -16,3 +16,7 @@ _describe the project a little_
 * source URL:
 * license: mit, bsd, gpl2+ , ...
 * platforms: unix, linux, darwin, ...
+
+**Prioritisation**
+
+Add :+1: for [issues you find important](https://github.com/NixOS/nixpkgs/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).

.github/ISSUE_TEMPLATE/unreproducible_package.md

@@ -29,3 +29,8 @@ nix-store --read-log $(nix-instantiate '<nixpkgs>' -A ...)
 
 (please share the relevant fragment of the diffoscope output here,
 and any additional analysis you may have done)
+
+
+### Prioritisation
+
+Add :+1: for [issues you find important](https://github.com/NixOS/nixpkgs/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).

.github/PULL_REQUEST_TEMPLATE.md

@@ -29,6 +29,11 @@ For new packages please briefly describe the package or provide a link to its ho
   - [ ] (Release notes changes) Ran `nixos/doc/manual/md-to-db.sh` to update generated release notes
 - [ ] Fits [CONTRIBUTING.md](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md).
 
+
+###### Prioritisation
+
+Add :+1: for [pull requests you find important](https://github.com/NixOS/nixpkgs/pulls?q=is%3Aopen+sort%3Areactions-%2B1-desc).
+
 <!--
 To help with the large amounts of pull requests, we would appreciate your
 reviews of other pull requests, especially simple package updates. Just leave a