Use buck rule for ReleaseNotes instead of Makefile

This eliminates the last Makefile in our code base.

Also change the section style within ReleaseNotes from asciidoc style to
asciidoctor style.

Also I feel that put images/link.png under ReleaseNotes and deal with
all the resource packing is too stupid, so I used the unicode emoji
instead of the picture ("🔗"). If this is too crazy, we can also use "#"
instead :) This also affects documentation rendering.

Other side effects:

1. The css of release notes switched from default asciidoc css into
default asciidoctor css.
2. The section anchors for ReleaseNotes/index.html changed from "2_13"
to "s2_13", because asciidoctorj is unhappy with anchors without
letters.

Change-Id: I4adf2ce090385cc6b699445012f10a009892aaac

ReleaseNotes/ReleaseNotes-2.0.21.txt

@@ -1,13 +1,11 @@
-Release notes for Gerrit 2.0.21
-===============================
+= Release notes for Gerrit 2.0.21
 
 Gerrit 2.0.21 is now available in the usual location:
 
 link:https://www.gerritcodereview.com/download/index.html[https://www.gerritcodereview.com/download/index.html]
 
 
-Schema Change
--------------
+== Schema Change
 
 *WARNING: This version contains a schema change* (since 2.0.19)
 
@@ -48,8 +46,7 @@ Apply the database specific schema script:
 ----
 
 
-Important Notices
------------------
+== Important Notices
 
 * Prior User Sessions
 +
@@ -106,8 +103,7 @@ Changing pool implementations is not required, c3p0 is still
 a supported provider.  I just want to make it clear that I no
 longer recommend it in production.
 
-New Features
-------------
+== New Features
 
 * GERRIT-189  Show approval status in account dashboards
 +
@@ -203,8 +199,7 @@ To enable the prior forging behavior, set `sendemail.from`
 to `USER` in gerrit.config.  For more details see
 link:http://gerrit.googlecode.com/svn/documentation/2.0/config-gerrit.html#sendemail.from[sendemail.from]
 
-Bug Fixes
----------
+== Bug Fixes
 
 * Fix ReviewDb to actually be per-request scoped
 +
@@ -276,8 +271,7 @@ The database schema incorrectly allowed two user accounts to have
 the same email address, or to have the same OpenID auth token.
 Fixed by asserting a unique constraint on the column.
 
-Other Changes
--------------
+== Other Changes
 * Start 2.0.21 development
 * Support cleaning up a Commons DBCP connection pool
 * Clarify which Factory we are importing in ApproveComma...