Fix sphinxify deprecation warnings with sphinx 2.2

[antonio-rojas]

With sphinx 2.2, sphinxify throws a deprecation warning:

/usr/share/doc/sage/common/themes/sage/search.html:22: RemovedInSphinx30Warning: To modify script_files in the theme is deprecated. Please insert a <script> tag directly in your theme instead.

Fix taken from rtd_theme [1]

[1] readthedocs/sphinx_rtd_theme@4a7dc08

CC: @kiwifb @timokau @isuruf

Component: documentation

Author: Antonio Rojas

Branch/Commit: e759f94

Reviewer: John Palmieri

Issue created by migration from https://trac.sagemath.org/ticket/29095

[antonio-rojas]

Branch: u/arojas/fix_sphinxify_deprecation_warnings_with_sphinx_2_2

[antonio-rojas]

Commit: e759f94

[antonio-rojas]

New commits:

e759f94

Fix "To modify script_files in the theme is deprecated" warning with sphinx 2.2

[antonio-rojas]

Author: Antonio Rojas

[antonio-rojas]

comment:3

With this fix, everything works fine at runtime with sphinx 2.2. Building is a different topic (#28856)

[jhpalmieri]

comment:4

When docbuilding I also see warnings

RemovedInSphinx30Warning: function based directive support is now deprecated. Use class based directive instead.

but those can be fixed on another ticket. (This is with Sphinx 2.3.1, which is what is available at https://pypi.org/project/Sphinx/#files.)

[jhpalmieri]

Reviewer: John Palmieri

[vbraun]

Changed branch from u/arojas/fix_sphinxify_deprecation_warnings_with_sphinx_2_2 to e759f94