Source code view should span full browser width

Currently the source code view spans only the central column width; it should use all the available width instead.

See for example what happens when resolving this SWHID

https://archive.softwareheritage.org/swh:1:cnt:c7143d7b433856579c589cdc5f786a1918413937;origin=https://gitorious.org/parmap/parmap.git;visit=swh:1:snp:78209702559384ee1b5586df13eca84a5123aa82;anchor=swh:1:rev:cc8915bceb7d05c2c645587f5eaaf0f6cb6080c6;path=/parmap.ml;lines=111-153/

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Migrated from T2456 (view on Phabricator)