Use lodash - remove global variables. Fixes #441 & fixes #501

Add lodash as dependency. It can be accessed with _.
Changes to utils are still open (see #441).


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Update by @LFDM

This is a premature PR, as other work is dependent on it.

We're introducing `lodash` here, which can clean up the code considerably in many places, especially for `arethusaUtil`.

We had a couple of global variables flying around - they are all injectable services now, provided by the new  `arehutsa.util` module. (cf. #501)

Only `arethusaUtil` remains as a global as of now. It's more work to clean this up -  which will happen soon in a follow-up to this PR.