Web server improvements

This pull request encompasses a number of significant under-the-hood and user-facing improvements to the web server.

Plots and merger trees in the halo/object view can now be resized dynamically using a split-screen divider
Data for producing plots is cached server-side to reduce the number of long queries to the database, especially when redrawing plots
Halos and other objects displayed in timesteps can now be sorted
Deleting a column from the timestep view or row from the halo view is now accomplished using an icon which is always available (rather than appearing while editing the property)
Errors during live property evaluation are now indicated by a warning triangle
Reconstructions are not attempted for the default list of properties in the halo view, greatly improving the speed of loading complex halos/objects
Miscellaneous under-the-hood improvements such as better scoping (use of let) in javascript

pynbody / tangos