Fix timeshift and movingaverage functions & improve docker-compose

[giom-l]

fix #128
This PR is to fix the moving average & timeshift function as proposed in #128
Credits for this go to drsoares

I tested this fix against version from 8.3.4 to 8.4.3 and it worked for me everytime.
I also improved the docker-compose part :

• Use a local volume to store grafana data (and prevent loss after container restart)
• Provision a grafana.ini config where the plugin is allowed as unsigned plugin (without it, it won't load in latest grafana versions)

If this PR is acceptable :

• the plugin should be properly signed to avoid having to state it as unsigned for it to work
• could you please release a proper grafana plugin to we can install it in grafana cloud and through grafana-cli (maybe add a CI part to do this automatically when a tag is created ? )

[ShilpaSivanesan]

@giom-l could you please pull the latest master and update PR

[giom-l]

Sure, since I kinda messed up with my repo, I created a new PR : #148