Migrating .html pages into markdown pages

* Migrate get-involved.html into get-involved.md
* Migrate static/how-it-works.html => content/page/how-it-works.md
* Migrate static/about.html => content/page/about.md
* Migrate apk.html
* Migrate building.html
* Migrate download.html
* Migrate copyright.html
* Migrate fdroid.html
* Migrating privacy.html
* Migrating hiring.html
* Migrating 2018-05-java-developer.html
* Migrating 2018-05-ux-designer.html
* Migrating 2017-beta-released-security-audit.html
* Migrating 2018-1.0-released-new-funding.html
* Migrating 2018-briar-1.1-released.html
* Migrating 2018-05-usability-consultant.html
* Remove redirect.html
* Move hirings to content/post/hiring/ folder
* Fix image source
* Use HTML on the _index.md page.
* Must still be adapted with hugo shortcode.
* /download: Use badges with html
* Add homepage description
* Fix a lot of links
* Fix layout for /fdroid
* Fix page layout for /apk
* README.md: Remove configuration part
* Add public/ to .gitignore