Add GitHub action to automatically rebase PR when a comment with '/rebase' is posted

[Bisaloo]

For security purposes, this is restricted to members of the epiforecasts organisation.

This PR follows the discussion in #107.

[codecov]

Codecov Report

Merging #114 (e5f1d10) into master (c091495) will increase coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
+ Coverage   48.74%   48.85%   +0.11%     
==========================================
  Files          18       18              
  Lines        1356     1357       +1     
==========================================
+ Hits          661      663       +2     
+ Misses        695      694       -1     

Impacted Files	Coverage Δ
R/utils.R	63.49% <0.00%> (ø)
R/sharpness.R	75.00% <0.00%> (ø)
R/pairwise-comparisons.R	42.14% <0.00%> (ø)
R/eval_forecasts_helper.R	100.00% <0.00%> (ø)
R/eval_forecasts_continuous_integer.R	69.64% <0.00%> (ø)
R/eval_forecasts_quantile.R	94.05% <0.00%> (+0.05%)	⬆️
R/eval_forecasts.R	66.07% <0.00%> (+0.89%)	⬆️

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[nikosbosse]

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