Configurable Forms for flows

[isuruf]

cc @lukeolson

A form description looks like https://github.com/isuruf/relate-sample/blob/91226393241cfed6457db533addfb2da6e83f645/forms/instant.yml
There are 5 types of fields. Text, Integer, Float, Choice, Hidden.
template_in and template_out are mandatory fields and they can be either Hidden or Text. If Hidden the values are fixed, but if Text, the user who fills the form can fill them.
If template_out=flows/asd.yml then the actual output would be flows/asd_20190930_022148_311577.yml where the time is appended.

There's a type for a form which is only flow for now. If there's a announce field and set to true and the type is a flow, then an InstantFlowRequest is created.

Flow created looks like

{% with id="20190930_022148_311577",
        field1="1",
        field2="A",
        template_in="flows/instant_flow.jinja",
        template_out="flows/instant_flow.yml",
        announce="True",
        created_time="2019-09-30 @ 02:21" %}
{% include "flows/instant_flow.jinja" %}
{% endwith %}

The templated flow template_in can use the form variables by their id and also have access to created_time and id. created_time is useful for instant flows that @lukeolson mentioned.

Let me know what you think.

[isuruf]

@inducer, let me know what you think. I'll add tests if you think there shouldn't be any major re-write.

[codecov]

Codecov Report

Merging #679 into master will decrease coverage by 0.48%.
The diff coverage is 74.29%.

@@            Coverage Diff             @@
##           master     #679      +/-   ##
==========================================
- Coverage   96.56%   96.08%   -0.49%     
==========================================
  Files          45       46       +1     
  Lines       11215    11459     +244     
  Branches     2084     2137      +53     
==========================================
+ Hits        10830    11010     +180     
- Misses        298      361      +63     
- Partials       87       88       +1

Impacted Files	Coverage Δ
course/content.py	96.7% <100%> (ø)	⬆️
relate/urls.py	100% <100%> (ø)	⬆️
course/constants.py	100% <100%> (ø)	⬆️
course/validation.py	99.83% <100%> (+0.01%)	⬆️
course/forms.py	65.21% <65.21%> (ø)

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[isuruf]

@inducer, I used this commit from inducer/relate-sample#12 which has flows/instant_flow.jinja file and it is included by the created flow using {% include 'flows/instant_flow.jinja' %}, but when I try this in tests the templating engine can't find this file. How can I make jinja2 see this file?