Truncate repr by default rather than changing to info view.

Truncate wide HTML repr rather than changing format.

Truncate HTML repr for DataFrames with MultiIndex-es

Add tests for long & wide Dataframe HTML reprs

Remove now-dead code for info display in HTML repr.

Fix HTML repr of wide tables with named indices

Make plain text repr follow HTML repr, truncating large DataFrames

Fix up tests for string repr

Update docs for DataFrame reprs

Add an option to switch back to info repr for large DataFrames
Add repr footer showing total number of rows and columns
Use to_string() to format DataFrames for clipboard.

Using str() has some unhelpful properties, especially when DataFrames
are large enough that the default repr does not show them in their
entirety. The defaults for to_string() are more helpful for the
clipboard.

Closes gh-5346
Update FAQ entry on controlling repr
Explain repr changes more prominently in release notes
Merge pull request #5550 from takluyver/long-repr-html

HTML reprs for large dataframes.