[explore] data panel should have fixed width

[junlincc]

note! this issue was not seen when the PR was last tested and merged. it might caused by conflicts

Screen.Recording.2020-12-18.at.8.50.01.PM.mov

minor cosmetic issue-changing dataset to one with longer name should not increase the width of left panel.
name should be truncated if not fit, tooltip should show when hovering
(this issue was not seen when first tested. )

to reproduce:

1. open the data panel by clicking the icon

2. change dataset to one that has long dataset name e.g. administrable_role_authorizations 3. see panel width change

Latest master