Fix language handling

Set the language before starting the asynchronous translation file
download. This ensures that other parts get the correct language.
Add introduction

* Add introduction page.
* Add images, texts and translations provided by Markus.
Add account registration form and change toolbar

* Add 'sign in' and 'register' fields to toolbar.
* Add account registration form.
* Registration via API.
Add track upload

* Add track upload functionality.
* Upload via API.

Remark:

* A clean error handling is missing.
* Deletion of tracks is not implemented.
Add .gitignore

* Ignore files and directories which should not be commited.
Change redirection on home site

Replace redirection to 'testdepth.openseamap.org'. Switch to
'#introduction' instead.
Remove unused template