add dropdown to experiment name, sample name, and cells

[byee4]

Let's try to add a dropdown menu of all existing experiment names, sample names, and cells for users to select from. The user should also be allowed to submit his/her own text which should then be added to the dropdown upon refresh.

For example, 'PRBF' will be entered as the experiment name when adding a new sample to the database. Once added, the input text should have a dropdown with two options: 'PRBF' and 'insert new experiment name'. We may also consider a 'default' field here too

[katherinemwong]

ideas:

1. use modelForm and render as a single field since there isn't a form built with the sample, experiment, cell fields of a fastq https://stackoverflow.com/questions/16055808/django-form-dropdown-list-of-stored-models
2. Brian suggested filter function

[katherinemwong]

first iteration: #33