Basic Visualization Frontend (web UI) + Server

[varisd]

Finished basic layout with an example figure type (metric tabulation):

• server correctly reads data from the TokEval output (output needs to be generated separately via CLI)
• supports language filtering using various language grouping (see tokeval/resources/language/languages.json)
• correctly identifying 2-D ("monolingual") and 3-D (paired "multilingual" metric resutls)

TODO:

• more figure types
• missing functionality (Load, Save, Export) buttons