Update LTS status of old loopback packages

[bajtos]

Once LoopBack 4.0 GA is released (#1744), we need to move LoopBack 3.x packages into Active LTS mode:

• Keep the latest version line in master, but mark it as "Active LTS". There will be no "Current" release line going forward. No new feature will be accepted, only bug fixes are allowed.

• The current Active LTS moves to Maintenance LTS, the version in Maintance LTS drops out of LTS.

• Add a short paragraph to README, explain why there is no Current version line and point users to LoopBack v4. Publish a new version to npmjs.org.

Packages to update:

• strong-remoting - Update LTS status strongloop/strong-remoting#456
• loopback - made it more clear that no new features are accepted - README: highligh Active LTS at the top strongloop/loopback#4029
• loopback-component-explorer - README: update LTS status strongloop/loopback-component-explorer#251
• loopback-swagger - README: update LTS status strongloop/loopback-swagger#133
• loopback-boot - README: update LTS status strongloop/loopback-boot#284
• loopback-cli - LTS strongloop/loopback-cli#78
• generator-loopback - add lts announcement strongloop/generator-loopback#398
• loopback-workspace - add lts announcement strongloop/loopback-workspace#537
• loopback-context - add lts annoucement strongloop/loopback-context#40
• loopback-phase - add lts annoucement strongloop/loopback-phase#28
• loopback-sdk-angular - add lts annoucement strongloop/loopback-sdk-angular#291
• loopback-sdk-angular-cli - add lts annoucement strongloop/loopback-sdk-angular-cli#79
• grunt-loopback-sdk-angular - add lts announcement strongloop/grunt-loopback-sdk-angular#37
• gulp-loopback-sdk-angular - add lts announcement strongloop/gulp-loopback-sdk-angular#21

[jannyHou]

Closing since all PRs merged.