WIP: Add event tracking to projects

[jsvgoncalves]

• For now only on Activity, but setting up PR for feedback.

In the current approach, the project table has a current_revision column. Each revision's a set of metadata, such as parent_revision, optional title , etc.

{
"metadata": {
  "revision": <this-revision-id>
  "parent_revision": <parent-revision-id>
  "title": "<optional>"
  "description": "<optional>"
  "authors": "<optional>" (maybe shouldn't be optional)
},
"data": {…}
}

Data is defined type and per object, e.g.:

"metadata": {…}
"data": {
  "activity": [deepdiff_1, deepdiff_2],
  "exchange": [deepdiff_3],
  "exchanges": []
}

Open questions:

• What is the directory structure like?
• How do we identify the objects' to change? Possibly:

  …
  "activity": [
  {id: <id>, diff: deepdiff_export},
  {id: <id>, diff: deepdiff_export},
  ],

• If we automatically save the revision on Activity.save(), how does the UX look like?