Allow to create subprojects for non top level projects

Hi there, new version of Vikunja allows to nest projects, which is great, but the dot menu still only allows to create projects on the top level, would be great to be able to create a subprojects for any project, the only way right now seem to be to create one on the top level and then drag and drop.

Change the PROJECT_INFINITE_NESTING_ENABLED variable in the index.html file (VIKUNJA_PROJECT_INFINITE_NESTING_ENABLED env if you’re using docker).

We limited this on purpose as we’re introducing nesting, infinite will probably be the default in one of the next releases.