Add a new endpoint for downloading usage metrics reports

[dmgaldi]

Overview

Now that the db permissions are in place, here's an endpoint to download usage metrics reports populated by jenkins job.

Results in dev:		study_id	num_users_full_download	num_users_subset_download
1	AVENIR-1	0	0
2	BRAZIL0001-1	0	1
3	INDIA0001-1	0	1
4	JILCOST-1	1	0

dataset_id	analysis_count	shares_count
1	DS_d6a1141fbf	60	0
2	DS_8e1f623542	25	3
3	DS_841a9f5259	15	0
4	DS_cb9893494e	9	0
5	DS_7c4cd6bba9	8	0
6	EDAUD_22881	6	6
7	DS_f5fc8ea2d2	6	0
8	DS_4d0fe88b6b	6	0
9	DS_450c62c7d0	4	0
10	DS_a885240fc4	3	0
11	DS_1d610aa72a	2	0
12	DS_def666ebb5	2	0
13	DS_6834a4c83b	2	0
14	DS_d70aacce42	2	1
15	EDAUD_25245	2	2
16	DS_6a93753102	1	0
17	DS_17191d35b9	1	0
18	DS_8e36901235	1	0
19	DS_f559aee789	1	0
20	DS_51b40fe2e2	1	0
21	DS_0086998c2f	1	0
22	DS_010e5612b8	1	0
23	DS_4902d9b7ec	1	0
24	DS_13de92039a	1	0
25	DS_2a6ace17a1	1	0

user_category	num_users	num_filters	num_analyses	num_visualizations
1	registered	8	21	45	65
2	guest	63	26	117	47

count_bucket	registered_users_analyses	guests_analyses	guests_filters
0	0	0	0	54
1	2	44	2	7
2	0	6	0	1
<=4	1	7	1	0
<=8	4	6	4	0
<=16	1	0	1	0
<=32	0	0	0	1
<=64	0	0	0	0
>64	0	0	0	0

registered_users_visualizations	guest_user_visualizations
0	2	30
1	0	24
2	0	5
<=4	1	4
<=8	1	0
<=16	3	0
<=32	1	0
<=64	0	0
>64	0	0