feat(client): support relative path (close #1492)

[Mister-Hope]

This adds:

• logic changes on isLinkHttp, now link starts with // is considered as http link
• logic changes on isLinkExternal, now link with protocols will all be treated as external
• relative link support for normalizeRoutePath resolveRoutePath and resolveRoute
• A new component <AutoLink> which can be used to render links in plugins and themes

[coveralls]

Pull Request Test Coverage Report for Build 8689059483

Details

• 15 of 54 (27.78%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.7%) to 41.014%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/client/src/router/resolveRoute.ts	0	1	0.0%
packages/client/src/router/resolveRoutePath.ts	0	2	0.0%
packages/client/src/components/RouteLink.ts	0	9	0.0%
packages/client/src/components/AutoLink.ts	0	27	0.0%
<!--	Total:	15	54	27.78%	-->

Totals
Change from base Build 8685728582:	-0.7%
Covered Lines:	689
Relevant Lines:	1715

---

💛 - Coveralls

[Mister-Hope]

@meteorlxy Could you review this? The downstream needs this.