Add proper templates in .github/ISSUE_TEMPLATE

Currently element-ios copies these implicitly from [`vector-im/.github`](https://github.com/vector-im/.github/tree/master/.github/ISSUE_TEMPLATE), but some of those templates (e.g. the bug report one) contain instructions that doesn't make sense for Element iOS:

```md
**Desktop (please complete the following information):**
- OS: [e.g. iOS]
- Browser [e.g. chrome, safari]
- Version [e.g. 22]
```

Maybe its better to introduce these to this repo specifically? Instead of letting them be copied from the `.github` repository.

(I also recommend changing the "bold" headers to actual headers, a-la https://github.com/vector-im/element-android/pull/2997)

Start in .github/ISSUE_TEMPLATE and compare the implicitly copied templates from vector-im/.github with the Element Android example linked in the issue. Define repository-specific templates for Element iOS, replacing desktop and browser instructions with relevant iOS fields and using proper headings; done means the templates are present locally and no longer contain irrelevant prompts.

Written by the indexing model from the issue text.