Enable to Download Multiple Objects in Horizon Object Store

Summary
==========
Currently the objects can only be downloaded one by one.
The goal of this blueprint is to enable downloading many objects at once.

===
Motivation
=============
Make it possible to download more than one object at once by horizon
interface.

===
Description
==============
To download many objects it will be necessary to compact all the objects
in a single zip file. To do this we would use the lib zipfile
(https://docs.python.org/2.7/library/zipfile.html).
Create a button to download these objects.
"Block" changes in the page to avoid errors during the compression.
If errors occur, show an alert message warning the user that the
download might fail.

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UX
======
[1]
The user will be able to download many objects from Horizon’s Object Store.
[2]
The user will select the desired objects for download by clicking
on the checkboxes.
[3]
The user will download the objects clicking on a single green button named
"Download".
[4]
The Download button will be shown when the first checkbox is selected.
[5]
The files downloaded will be saved into zip file named "Objects <datetime>.zip"
[6]
When the files are being compressed a modal with a loading animation will be
shown on the page disabling the user input.
[7]
A warning message will be shown if the user is trying refresh or close
the page during compression.

===
Testing
==========
[1]
Test if the files were compressed correctly
[2]
Test if the alert message is shown when a refresh is trying or change the page.
[3]
Test if it’s possible for the user to click on a link in the page.
The test fails if the user is able to do so.
[4]
Test if the green button is shown when at least one object is selected
by the checkboxes.
[5]
Test if the loading modal is shown when the files are compressing.

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Outside Dependencies
=======================
N/A

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Requirements Update Required
===============================
N/A

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Doc Impact
=============
N/A