deposit: fieldlist separation

During development of import functionality for deposit in inspire, I found out that it would be easier to have a more direct access to former fieldlist jQuery plugin. The plugin takes care of dynamic field lists like the list of authors in the deposit form. 

After the RFC made here: https://github.com/inveniosoftware/invenio/issues/1775 I decided to separate the plugin to an external file. Its name was changed to "DynamicFieldList", what is a proposition, and may better describe the functionality.

The first commit https://github.com/kneczaj/invenio/commit/5579d8d7ad1461f6bbb7cf6f486e4ed6530db91a was tested sligthly - without copy-paste functionality, and it works.

As there is little documentation, I had some problems with using original `append_element()` function of the plugin with value initialization, also described in the RFC above. I decided to make a function `set_fields_values()` which seems to be more simple and straightforward. I would like to substitute `update_element_values()` with it after tests. I am also not sure if there's a need of having append_element() function with initialization inside. In the second commit, I removed this functionality.

Can you tell me what do you think about the changes? If they are ok, I will move `set_fields_values()` code to `update_element_values()` and adapt the rest of code to it.

@jirikuncar @lnielsen-cern
1. The selector which is the first argument of a jQuery plugin is a list of the elements. When a jQuery plugin is run on a selector with multiple objects it is supposed to apply the plugin to every single object. I see three possible ways of returning the result:
a) return the result of first element of the selector only
b) return the list of the results
c) return single element when there is only one in the selector and a list when there is more.

Solution a) is the most common, although it skips all the elements in the selector beside the first one.
Solution b) is the case applied here.
Solution c) is ambiguous so it may be bug-prone.

The case applied originally in `fieldlist` didn't expect more than one element in the selector. I can say that it was the most similar to a) but it would  have unexpected behavior after applying on a selector with multiple results.

2. We want just the first one because the selector is get from `$.each` so it always returns just one object.

The plugin may be also applied instead of using `$.each` in this way on all the fields at once:
```javascript
field_lists = $(selector).dynamicFieldList(opts);
```
but then the result is an array not an object.