search: make "repository:" condition work case-insensitively as expected

Before this revision, "repository:" condition at searching for "Commit
messages" never shows revisions in a repository, of which name uses
upper case letter.

Using ID for "repository" of CHGSETS_SCHEMA preserves case of
repository name at indexing. On the other hand, search condition
itself is forcibly lowered before parsing.

- files in repository "FOO" is indexed as "FOO" in "repository" field
- "repository:FOO" condition is treated as "repository:foo:

Then, indexing search itself is executed case-sensitively. Therefore,
"repository:FOO" condition never show revisions in repository "FOO".

But just making "repository" of CHGSETS_SCHEMA case-insensitive isn't
reasonable enough, because it breaks assumptions below, if there is
case-insensitive name collision between repositories, even though
Kallithea itself can manage such repositories at same time.

- combination of "raw_id" (= revision hash ID) and "repository" is
unique between all known revisions under Kallithea

CHGSETS_SCHEMA assumes this.

This unique-ness is required by Whoosh library to determine
whether index table should be updated or not for that repository.

- searching in a repository shows only revisions in that repository

Before this revision, this filtering is achieve by "repository:"
condition with case-preserved repository name from requested URL.

To make "repository:" search condition work case-insensitively as
expected (without any violation of assumptions above), this revision
does:

- make "repository" of CHGSETS_SCHEMA case-insensitive by
"analyzer=ICASEIDANALYZER"

- introduce "repository_rawname" into SCHEMA and CHGSETS_SCHEMA, to
ensure assumptions described above, by preserving case of
repository name

"repository_rawname" of SCHEMA uses not ID but TEXT, because the
former disable "positions" feature, which is required for
highlight-ing file content (see previous revision for detail).

This revision requires full re-building index tables, because indexing
schemas are changed.