Increase backend github/models coverage

[arkid15r]

Is your feature request related to a problem? Please describe.
Improve the test coverage to 75%+

apps/github/models/common.py                                                       36      8      4      0    70%   36, 41, 45-65, 79
apps/github/models/issue.py                                                        95     39     14      0    51%   92, 97, 108, 113, 117-143, 147-153, 157-167, 171-177, 182, 188, 193-203
apps/github/models/label.py                                                        34     16      6      0    45%   24, 29-41, 46, 51-61
apps/github/models/managers/issue.py                                               13      3      0      0    77%   15, 37, 48
apps/github/models/mixins/issue.py                                                 74     24      0      0    68%   12, 21, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135
apps/github/models/mixins/organization.py                                           8      2      0      0    75%   12, 17
apps/github/models/mixins/repository.py                                            31     10      0      0    68%   10, 15, 20, 25, 30, 35, 40, 45, 50, 55
apps/github/models/organization.py                                                 32     17      6      0    39%   27, 31-41, 46, 51-61
apps/github/models/release.py                                                      41     18      6      0    49%   39, 43-64, 69, 74-84
apps/github/models/repository.py                                                  120     55     24      0    45%   102, 107, 117, 122, 127, 143-234, 247-264
apps/github/models/repository_contributor.py                                       33     15      6      0    46%   35, 42-50, 55, 60-75
apps/github/models/user.py                                                         30     16      6      0    39%   22, 26-38, 43-53
apps/github/utils.py                                                               30     12     10      0    60%   12, 44-45, 50-63

[yashpandey06]

@arkid15r can i work on this ?

[yashpandey06]

Update : was working on production related pr ..will be focusing on this ..some of the files are done ...will raise PR soon .