Adding download code for ARCT, MCTest, MCTACO, MuTual, and QuAIL

[claravania]

This PR adds download code for the following tasks: arct, mctest160, mctest600, mctaco, mutual, mutual_plus, and quail. Besides quail which uses HF datasets, all tasks use direct download links.

Tested using the following command:

export PYTHONPATH=jiant/
python jiant/jiant/scripts/download_data/runscript.py download --tasks [TASK_NAME] --output_path [OUTPUT_PATH]

[jeswan]

One concern about QuAIL (see comment). Will approve after this concern and formatting is addressed.

[codecov]

Codecov Report

Merging #1258 (a3cfd5b) into master (9a45712) will decrease coverage by 0.34%.
The diff coverage is 7.69%.

@@            Coverage Diff             @@
##           master    #1258      +/-   ##
==========================================
- Coverage   55.86%   55.51%   -0.35%     
==========================================
  Files         149      149              
  Lines       10825    10903      +78     
==========================================
+ Hits         6047     6053       +6     
- Misses       4778     4850      +72     

Impacted Files	Coverage Δ
jiant/scripts/download_data/constants.py	100.00% <ø> (ø)
...pts/download_data/dl_datasets/hf_datasets_tasks.py	100.00% <ø> (ø)
jiant/tasks/lib/quail.py	77.77% <ø> (ø)
...t/scripts/download_data/dl_datasets/files_tasks.py	7.56% <7.69%> (+0.02%)	⬆️

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[claravania]

@jeswan I've fixed the QuAIL and reformat issues.