Added Mentorship Relation Tasks feature

[isabelcosta]

Description

Completed a simple version of the Tasks feature
Tasks feature is a JSON object part of TasksListModel which has a 1:1 relation with MentorshipRelationModel.
I'm not allowing for users to create a task which is already achieved.

A task entity will have the following format. This is the JSON response returned by the List all Tasks API:

[
  {
    "id": 3,
    "description": "sloa loa loag",
    "is_done": false,
    "created_at": 1534132997.472032,
    "completed_at": null
  },
]

Request Body:

{
    "description": "sample text"
}

New APIs:

Note: You may notice that I deleted some repeated code from tests, this is related to the setup of Mentorship Relation tests, there was a lot of repeated setup that I moved to a file which has this common setup. I did this in this PR because I was already changing this files to adapt to the new changes on MentorshipRelationModel.

Fixes #82

Type of Change:

• Code

Code/Quality Assurance Only

• New feature (non-breaking change which adds functionality pre-approved by mentors)

How Has This Been Tested?

Created extensive tests for each module: DAO, database model and API
Tests can be still improved, although I think the ones I developed are enough for now.

Checklist:

• My PR follows the style guidelines of this project
• I have performed a self-review of my own code or materials
• I have commented my code or provided relevant documentation, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• Any dependent changes have been merged
• Update Swagger documentation and the exported file at /docs folder

Code/Quality Assurance Only

• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[isabelcosta]

@m-murad you suggested more fields in a weekly meeting:

{
           "id": 1,
           "create_on": 456789098765,
           "create_by": "Someone 1",
           "title": "Title 1",
           "description": "Description 1",
           "done": true,
           "completed_on": 12345678,
           "completed_by": "Someone else 1"
}

Also Current Mentorship Relation API does not return tasks yet, but I think this can be added in a later PR. This will be helpful for Current Relation screen on the android application.

@systers/maintainers-mentorship-android thoughts on this? 1º Extra fields; 2º Add or not the tasks response together with the list relation API? Do this on this PR or another?

[Dilu9218]

I think it would be better to do it in a separate PR

[isabelcosta]

@Dilu9218 I think too. just because its an enhancement, even though necessary, it is an enhancement, this PR has the basis of the feature.

[r00pal]

@isabelcosta do the enhancements in a separate PR taking this as the basis. I'm merging this one.

[isabelcosta]

OK thank your for merging it @roopalJazz. Now I should create the PR for merging gsoc18-code into develop?