Use properties file to map file extensions to CM3 mime types

Use a properties file to map file extensions to the mime types
used for syntax highlighting in codemirror.

Using a properties file makes it easier to update the mapping
and keep the lines sorted.

Change-Id: Ic84c01b5fd783ffbc59873daa5e758317409f163
(cherry picked from commit bac7d1c053c8a5cdcf84c9628e29650df9fa5df2)
SideBySide2: Add syntax highlighting for 18 more languages

Include more languages into the CM3 mapping supported by Gerrit:

* coffeescript
* d
* diff
* dtd
* erlang
* gas
* gfm (GitHub Flavored Markdown)
* haskell
* lua
* markdown
* php
* pig
* r
* scheme
* smalltalk
* tcl
* verilog
* yaml

Change-Id: Ie3cc3657183a475fb71a71e0fe35be848deb985f
(cherry picked from commit 5478148f6f25db215424f51d6a5e88f7bf78f164)
2.9 release notes: Add warning about download commands

Add a warning that reminds administrators that the download-commands
plugin must be installed for getting download-commands on the new
change screen. This warning was already included in the 2.8 release
notes but it makes sense to add this information to the 2.9 release
notes as well. Some people may not have been interested in the beta
version of the new change screen in 2.8 and hence they may have
ignored the need for installing the download commands plugin.

In addition provide more detailed instructions about how the
download-commands plugin can be installed.

Also add a warning about upgrading the plugin, since the 2.8 version
of the download-commands plugin doesn't work with Gerrit 2.9. Explain
that interactive init must be used to upgrade the plugin.

Change-Id: I2101e0a3974ddb8d1fdf198941edc99305351650
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
Clarify new installation vs. upgrade of plugin in 2.9 release notes

The download-commands plugin can be either newly installed, or
upgraded, when installing Gerrit 2.9

Make this a bit clearer in the release notes.

Change-Id: If6feeb5122d8bdc4272d086f540485dc66a0e44d
RebaseChange: Fix typo

Change-Id: I8f8fd169ce258191b2f1fcc70e0673edb7eadc9d
Minor updates in 2.9 release notes

- The "Schema Update" section includes several warnings not related
  to schema updates.  Rename it to "Important Notes".

- Move note about removal of SQL search into "secondary index"
  section.

- Add missing periods.

Change-Id: I8ae0cc3bb8d99c711cd96812a3697b46be2a21f0