Upgrade to spring boot 2

[dianw]

Merge this PR once the external oauth2 provider login feature are done

[codecov]

Codecov Report

Merging #79 (e58d11c) into master (af254d6) will decrease coverage by 4.58%.
The diff coverage is 76.00%.

@@             Coverage Diff              @@
##             master      #79      +/-   ##
============================================
- Coverage     86.64%   82.06%   -4.59%     
+ Complexity      476      410      -66     
============================================
  Files            89       84       -5     
  Lines          1400     1232     -168     
  Branches         30       25       -5     
============================================
- Hits           1213     1011     -202     
- Misses          168      205      +37     
+ Partials         19       16       -3     

Impacted Files	Coverage Δ	Complexity Δ
...theskeleton/core/data/AuditInformationAuditor.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...denergic/theskeleton/core/data/S3ClientConfig.java	93.33% <ø> (-2.59%)	6.00 <0.00> (-5.00)
...gic/theskeleton/core/data/UTCDateTimeProvider.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../codenergic/theskeleton/core/web/WebMvcConfig.java	100.00% <ø> (ø)	4.00 <0.00> (ø)
...denergic/theskeleton/core/web/WebSocketConfig.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...skeleton/registration/RegistrationServiceImpl.java	81.08% <ø> (+2.13%)	7.00 <0.00> (ø)
...va/org/codenergic/theskeleton/user/UserMapper.java	100.00% <ø> (ø)	1.00 <0.00> (ø)
...heskeleton/core/mail/ThymeleafTemplateService.java	84.61% <84.61%> (ø)	4.00 <4.00> (?)
.../codenergic/theskeleton/core/mail/EmailConfig.java	100.00% <100.00%> (ø)	3.00 <1.00> (+1.00)
...c/theskeleton/core/security/WebSecurityConfig.java	100.00% <100.00%> (ø)	5.00 <0.00> (ø)
... and 8 more

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