Add narrative to Tales

[Xarthisius]

This PR introduces Tale's narrative, i.e. a set of scripts/files/notebooks that is going to be injected into the working directory upon Tale instantiation. In case of '.ipynb' and '.Rmd' files we could default to opening them after start in Jupyter and Rstudio respectively.
This feature will be divided into three stages:

1. Adding narrative support in Tale model (this PR)
2. Injecting narrative content into Tale Instance (PR to gwvolman)
3. UI modification, that would allow users to use narrative (PR to dashboard)

Stage 3 is outside of scope of v0.5 milestone. However, 1 and 2 will be used to address whole-tale/gwvolman#14

[codecov]

Codecov Report

Merging #131 into master will increase coverage by 0.8%.
The diff coverage is 96.29%.

@@            Coverage Diff            @@
##           master     #131     +/-   ##
=========================================
+ Coverage   77.98%   78.78%   +0.8%     
=========================================
  Files          20       20             
  Lines        1649     1669     +20     
=========================================
+ Hits         1286     1315     +29     
+ Misses        363      354      -9

Impacted Files	Coverage Δ
server/schema/tale.py	100% <ø> (ø)	⬆️
server/rest/tale.py	87.75% <ø> (ø)	⬆️
server/constants.py	80% <100%> (+0.51%)	⬆️
server/models/tale.py	89.39% <96.15%> (+9.74%)	⬆️

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[ThomasThelen]

LGTM This should be a really nice feature