Update issue templates

Add "installation problem", and while I'd rather not have larger proposals on the tracker, it would still be useful to list small requests where it's objectively clear what the resolution is.

.github/ISSUE_TEMPLATE/installation-problem.md

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+---
+name: Installation Problem
+about: Problem compiling or installing Idris 2
+title: ''
+labels: Installation Issue
+assignees: ''
+
+---
+
+Please check INSTALL.md and README.md to ensure you have all the required dependencies. In particular, you need at least Idris 1.3.2 to build Idris 2, and a version of Chez Scheme compiled with threading support (this is the default in most distributions) in order to run the tests successfully.
+
+Otherwise, please describe the problem, including any relevant parts of build logs, and describing as much about your environment as possible.

.github/ISSUE_TEMPLATE/large-feature-requests-and-proposals.md

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+---
+name: Large feature requests and proposals
+about: What to do if you have a feature request or proposal
+title: ''
+labels: Proposal (please add to wiki/mailing list)
+assignees: ''
+
+---
+
+Experience with Idris 1 suggests that larger feature requests and proposals tend to get buried among the bug reports in the issue tracker, even if we tag the effectively. Partly this is because there's not many people working on Idris full time (it's about half a person at the moment), but also, it's because it's not objectively clear how to resolve such issues.
+
+So, instead, please add the details on the wiki, at https://github.com/edwinb/Idris2/wiki/Contributions-wanted, which I hope will be more discoverable. It's also a good idea to suggest it on the mailing list and/or the IRC channel, which are good places for more detailed discussion.