Upgrade dataTables library to 1.10.20

[shrektan]

Closes #763

• improve update_DT.R
• remove all the deprecated files (the theme files other than bootstrap should not be used as well but we may need them in the future when we want to support more themes)
• update js/css source files
• bumper DataTablesVersion
• add the new extension searchpanes
• make sure searchpanel ext works by creating an example (note it only works for the client-processing mode as mentioned in https://datatables.net/blog/2020-01-13 )
• make sure all the example can be run well
• rebuild the site (see Rebuild site (and update Extension and Plugins) #757)
• update NEWs and bumper DT version

SearchPanes Example

DT::datatable(
  iris,
  options = list(dom = 'Pfrtip', columnDefs = list(list(
    searchPanes = list(show = FALSE), targets = 1:3
  ))),
  extensions = c('Select', 'SearchPanes')
)

[yihui]

Thank you very much!

[saheedb]

Why does targets = 1:3 specify columns 2 and 5 in iris? I'm unable to get searchPanes to just show the columns I'm interested in--I'm not sure how the targets parameter is supposed to work.

[shrektan]

Why does targets = 1:3 specify columns 2 and 5 in iris? I'm unable to get searchPanes to just show the columns I'm interested in--I'm not sure how the targets parameter is supposed to work.

1. It's not column 2 and 5. The columns in the searchPane are 4th and 5th.
2. searchPanes = list(show = FALSE), targets = 1:3 means "do not show 1:3 in the searchPanes" (it's show = FALSE not TRUE). By default, the searchPanes will display all the columns so you are supposed to specify the "hidden" columns.
3. For more info please look at the offical example