add new field in config, check upload size

[Repumba]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

What is the current behaviour?

Users can upload as big file as they want and administrator doesn't have any tools (inside MWDB) to prevent that.

What is the new behaviour?

Administrator can configure limit for uploaded files
Checking file's size is not precise. In order to simplify the code, I check Content-Length field in request header. This value is bigger that actual file size by +/- 500B.
In my opinion this error margin is acceptable - in most scenarios the limit will be set to megabytes, so even if the other values in request were 1 kB, the margin of error would be less that 0.1%

Test plan

Change values of max_upload_size in config and try to upload a file.

Closing issues

closes #9

[Repumba]

Questions to discuss:

1. Should max upload size apply to configs and text blobs?
2. Should we add a capability to omit the limit?