Prep storage-1.2.0 release.

[tseaver]

Supersedes #3529.

Requires merge of #3526 to bump core to 0.25.0.

Draft release notes:

google-cloud-storage-1.2.0

• Update google-cloud-core dependency to ~= 0.25.
• Document that Bucket.lifecycle_rules, Bucket.storage_class are settable properties. (PR Expose that settable properties are so. #3472, issue Setting lifecycle rules totally undocumented #2610).
• Add Bucket.labels property. (PR Add 'Bucket.labels' property. #3478, issue Add support for bucket labels #3473)
• Remove mention of no-longer-chainable ACL methods. (PR Scrub mention of no-longer-chainable ACL methods. #3428, issue Chaining ACL grants on Storage does not work. #3181)
• Restore support for retries in storage uploads. (PR Add back support for retries in storage uploads. #3378)
• Drop internal usage of rewind in Blob.upload_from_string(). (PR Dropping internal usage of rewind in Blob.upload_from_string(). #3365)
• Change Blob.upload*() and Blob.create_resumable_upload_session() methods to use google-resumable-media. (PRs Changing over Blob.upload*() methods to use google-resumable-media. #3362, Factoring out some Blob helpers. #3357)
• Enforce explicit UTF-8 blob name. (Enforcing explicit UTF-8 blob name. #3354)

---

Not included in notes:

• Revert "Fix 'broken' docs build. (Fix "broken" docs build. #3422)" (Revert "Fix "broken" docs build. (#3422)" #3439)
• Vision semi-GAPIC (Vision semi-GAPIC #3373)
• Re-organize the documentation structure in preparation to split docs among subpackages (Re-organize the documentation structure in preparation to split docs among subpackages #3459)
• Fix "broken" docs build. (Fix "broken" docs build. #3422)
• Clean up a documentation for the storage blob module. (Clean up a documentation for the storage blob module. #3376)

[dhermes]

@tseaver I made one tweak (about settable properties) to the notes, but LGTM otherwise.

Maybe rebase this PR on top of #3526 so tests will pass?

[tseaver]

@dhermes I'll just kick off a rebuild.